They're rushing to get this through Parliament before Trump is inaugurated, I suppose before Marco Rubio can be confirmed and the phone lines between Foggy Bottom and Whitehall get set on fire.

That doesn't surprise me.

The UK has been taking a diplomatic beating on this for a while. Their control of the Chagos Islands came about in a way that looks....bad today. The UN set out a bunch of rules that were supposed to cover decolonization - namely that in granting independence to former colonies the former conquering powers weren't supposed to keep choice goodies for themselves. The UK didn't do that with the Chagos Islands. Instead, they kept them for themselves and forcibly removed the entire native population so they could have a better site for a military base.

That sort of thing might have been a little more par for the course back in the 1960's, but it's a lot harder to justify these days. Considered a little less "realpolitik" and a little more "crime against humanity" to do this type of move. So the International Court of Justice ruled that UK had violated international law when they did all this, ordered the UK to give the area back to Mauritius, and the UN passed a resolution ordering them to comply. They haven't, but they've been paying a real diplomatic toll ever since:

BBC: Chagos Islands dispute: UK accused of 'crimes against humanity' by Mauritius

When you're trying to encourage all of the nations in that region to side with Team West rather than Team China, the past depredations of the UK were always going to be a negative - but them holding onto this territory in the fact of an ICJ ruling and a UN resolution telling them it was completely unlawful makes things even worse. Not exactly a selling point for the West being the ones who are on the Side of Law against those nasty Chinese. And an even harder position to take in light of the furious (and justified) condemnation of Russia's grab of the Ukraine - if you're out there saying that just because seizing territory would advance your national security interest you can't just go out and take it, then why is the UK still allowed to unlawfully hold onto the Chagos?

Mauritius offered the UK a way out of this bind - return sovereignty, but keep your base. We'll give you a 99-year lease, but you have to pay us for it. It's not a bad deal for the UK, since they get to solve their diplomatic problems and a black eye, while keeping the base. It's a great deal for them.

It's a very bad deal for the U.S. - because we weren't the ones getting raked over the coals at the UN. It's very easy for Rubio to tell the UK that we'd prefer that they keep their todger stuck in the meat grinder indefinitely, because it's not our todger getting mangled. But the UK is quite reasonably trying to minimize the damage they're suffering. "UK First," after all!