Hmm, yeah that's a tough situation for you. In your position, I would be an asshole and accept the UK offer by their deadline, but then if given a better offer I would jump ship and take the US offer.
But yes, as soon as I got a good offer I withdrew all my other applications to the schools below it in my preference list. My classmates are applying to overlapping schools with similar backgrounds (since we went through the same program) and I didn't want to cockblock them, so I definitely withdrew from those schools. I guess it's also a courtesy to other strangers out there such as people on this forum.
I don't know if others withdraw their applications after getting accepted to a school they want to attend, but they really should!