Frankly I'm a little confused about what this couple wants exactly, but maybe I'm just not up on current UK marriage laws.
So I'm assuming that 'civil partnership' is a legal arrangement reserved for same-sex couples because actual marriage isn't an option for them? Are we in the US actually ahead of the UK on this? Do you not have the concept of 'civil marriages' in the UK- legally recognized marriage performed by civil authorities, outside of any religious affiliation? Is there some prohibition against them cohabitating with some sort of binding agreement? What the hell is their point anyway?

I was confused by this, too. Reading between the lines, I'm guessing there are in-built and unequal assumptions about gender in the UK legal code regarding marriage that are not present for "civil partnership." As far as I know, similar assumptions are not built into the US legal code, but I wouldn't be surprised if they once were. The word generally used in US law is "spouse" not "husband" or "wife." Plus a marriage is a marriage, whether done through a member of the clergy or by a civil servant. We do have "civil unions" which are not the same as marriage, in that they are State based, and do not carry the same Federal, interstate rights of marriage.

Full disclosure: I am not a lawyer and may be purely talking out of my ass.

I believe you're generally correct. Legal marriage offers certain universal legal guarantees, which is mainly why SCOTUS made it available to same-sex couples. Otherwise they'd have to depend on domestic partner status that does vary widely from state-to-state. Now that gay marriage is legal I don't know that 'civil unions' are still at thing at all. People can still shack up and live committed unmarried lives, and they may enjoy some degree of legal protection (depending on your state's laws on the matter), but only marriage fully guarantees this. I have several friends who are in such partnerships, they have kids and own property together but for whatever reason never bothered officially tying the knot. Of course remaining unmarried may be advantageous if say, your partner has a lot of debt that you don't want to potentially incur. Otherwise, if you're going to enter into that kind of commitment with someone, it just seems reasonable to have some kind of contractual arrangement, and legal marriage is just kind of an easy, one-off way to do it.

I see your possibly talking out of your ass, and raise you thus:

I've done no research on this, but would assume it's a symbolic thing more than a legal thing. Historically marriage isn't about "two people who love each other very much", it's about land and property ownership. A dowry is close to a payment by the family of the female to the male, in exchange for marrying the female. This of course being back in the days when women of rich families didn't inherit, even if they're older than their male siblings. Women seen as property to be married off with a dowry or a bride price.
Similarly, in religious terms, "marriage" is tied to Christianity, and is between a man and a woman.
Civil partnerships which, iirc, were allowed in the UK before gay marriage, were a compromise for gay couples. It allowed them to have the same rights, but without insulting those who saw "marriage" as a primarily religious service. Except of course, that few childrens books talk about beautiful civil partnerships, so gay people still wanted to get married.

So now we have a couple, almost certainly atheists and probably feminists (or similar), wanting to have all the legal rights given by marriage, whilst taking a symbolic step away from things they don't want to implicitly endorse. But they aren't same sex, so they can't have a civil partnership. Enter: legal battle.

There's caveats I've glossed over, but in general I think that's it.

Except marriage in the US is really just a legal contract, affording mutual protections to both spouses and their offspring. It need not have any religious connotations at all.
I for one cannot speak to whatever ancient Druid customs and expectations come with official marriage in Old Blighty.
My assumption is that most married British couples are like Andy and Flo Capp, and maybe that's enough of a reason to disdain the institution.

I'll jump in with some well researched information about another country entirely. In New Zealand we had civil partnerships before gay marriage. So, marriage was still just for straight people, and the gays could have civil union (all the legal stuff of being married, but still not a 'real' marriage in many eyes, and as a sop to those people non-married couples could not adopt, although non-married individuals could - it's confusing and stupid, I know). We did it slightly differently in that anyone who wanted a civil union (even straights) could have one. And a lot of people did because, as Spyike says, it's all the legal stuff, but with none of the baggage and expectations.

But a lot of people (including me and my now husband) thought it was a bit of a second-class idea. New Zealand isn't very religious, most marriages are secular and it seemed very much like a second-rate thing, an acknowledgement that there was a relationship, but that it wasn't quite as good as the 'real thing'. That the government was still giving out prizes for matching up the 'right' combination of genitals and it wasn't what we wanted. I'm a woman who likes dick, and the dick is it's own reward, I don't need a pat on the back from the church or the state for that. I can fully see this couple thinking something similar. And being prepared to fight about it.

Hopefully that feeling that civil partnerships are second rate to marriage will eventually go away, since it's only/mostly a cultural thing. It doesn't help that 'marriage' sounds grand, whereas 'civil partnership' sounds like beaurocracy, but still.

It was the civil partnership that went away, in the end. In 2013 the Marriage Equality Act was passed and now it's all just marriage ("marriage means the union of 2 people (consenting adults), regardless of their sex, sexual orientation, or gender identity"). Marriage is what couples choose to make it. The only political party running on a platform of overturning the Act didn't get enough party votes (5% nationwide) to get into Parliament.

We got married in a fully secular ceremony, I wore blue, both my parents walked me down the aisle, we said our own vows and we had a sculpture cast that had two parts and we joined them together instead of exchanging rings.

From the point of view of the state there should be no difference. Either you're legally bound and committed or you aren't.

I was going to say that Marriage Lite without the same obligations in divorce sounds nice, but then I remembered that we covered this with prenups. Divorce is such a high risk and can be so expensive I doubt I would marry without a legal agreement on what happens in the event of divorce.

Problem is, let's say you shack up with an partner for several years. Maybe you buy a house together and have a kid. You still have to deal with a crazy legal mess if you break up, only without the legal guidelines of actual marriage. In a situation like that there's no such thing as 'marriage lite'. Really this situation works best for couples who are both piss-poor and childless.

That didn't make me less afraid of commitment at all! At least we'll always be childless. Unless you count pets. Unfortunately my partner and I both do.

It's good I have a scotch right now or I'd be in my car skipping town.

We sort of have it. If a couple is in a committed relationship they have the same protections as a married couple in the case of a break-up. Marriage law with regard to property applies. But, unlike marriage, if the break up is amicable, or at least the property and custody arrangement is uncontested, then they can just walk away from each other. Alimony does not apply.

Being aware of where this stuff comes from is, for a lot of people, interesting and important. For example, Father's Day was fabricated to balance out Mother's Day, which is originally about returning to your "mother church", i.e., the church you went to as a child (although very few people know that now, I think). It had nothing at all to do with making someone breakfast in bed, but things change. And that history is equally true for Americans celebrating Mother's Day. I'm not sure where I'm going with this, I just like that Mother's Day fact. "Siblings Day" can %100 fuck off.

A agree with your unresearched assumptions. I think the question is how much of those assumptions have been codified into English law? That's what I was talking about when I mentioned the use of "spouse" as opposed to "husband" and "wife" in US law. I'm guessing that the British laws are a lot older than US laws, and take that much more to overcome their inertia. I wouldn't be at all surprised if there are all sorts of gender specific references in them that have yet to be stricken from the books. The civil union laws, on the other hand, being much more recent, probably lack those built in assumptions, not just because they were designed more particularly for same gender couples, and a few, more, commas, just, for, the, hell, of, it.