Because the UK is the closest to the US in terms of geopolitical alignment, especially after brexit. Together with the US they've been the strongest supporters of NATO co-belligerence in the Ukraine/Russia war, even within NATO itself, let alone Europe. I might also cite the Liz Truss phone hack, but it hasn't really been confirmed.
So you say it for exactly the reasons I thought you would. (Although I think Poland is, again, more vocal in NATO support for Ukraine than UK.)
Of course, circling back to the topic at hand, the problem with blaming the US for the pipeline explosion is that it makes no fucking sense for the US to blow it up.
Germany had already committed to weaning itself off of Russian gas and Nord Stream as soon as Russia invaded Ukraine. There has been no real political support for backsliding on this decision, so the best possible scenario for the US were it to blow up the pipeline would be... a continuation of current policy; there's no real gain to be had by blowing it up. The risk of blowing it up is an uptick in criticism against the US for being foreign interventionists, resulting in increased political pressure to force the EU to force Ukraine to accede to substantial losses to Russia. Even if the US were stupid enough to desire a complete cessation of gas, the way they would want to go about it would be something that minimizes the chance of political blowback--in other words, the operation would want to do stuff that keeps it out of headlines, and a pipeline explosion is a pretty dramatic affair that is completely at odds with how to achieve your political ends.
Another element to consider is that it's very well-known that there is a gas shortfall problem in 2023 without Nord Stream. There's not enough LNG import capacity to fully replace it until about 2024, which means if it's cut off entirely, there is going to have to be some form of politically painful rationing going on to make it through the winter of 2023-2024. If your goal is to sell more gas, creating a situation that forces people to use less gas is counterproductive. And, again, as popular appreciation of gas shortages come about, it's going to create more pressure against support for Ukraine in the Russia-Ukraine war.
If you look at it instead from the perspective of Russia, it makes a lot more sense. Russia already decided to stop exporting gas through Nord Stream about a month before the explosions, so it isn't going to be losing any revenue if it blows it up. In lieu of achieving military success on the battlefield, its theory of victory seems to have shifted in the fall towards making Ukrainians' and Europeans' lives as miserable as possible to force Ukraine to the negotiating table just to make the war stop. A dramatic explosion pushes the gas shortage issue into the headlines, and has enough plausible deniability to avoid political blowback. Especially because there's a decent cadre of people who will blame everything on US interventionism (and that's more or less official Russian line anyways--the Russia-Ukrainian war is the fault of the US, not Russia), even without any real evidence. Really, the strongest evidence that it's not caused by Russia is that it makes more strategic sense than Russia has shown itself capable of in the war to date.
The only thing that makes sense to me is it was blown up to force the Russians to expend resources protecting their infrastructure inside Russia. So likely Ukrainians did it while the Baltic states just looked the other way.
