I've wondered how this gap persists. It seems like UK companies would be competing in the same global market as the US firms. In many cases, the US companies have offices in the UK. Does Google's London office pay 30% less than Google's New York office?

Not really... I made marginally more in the US than UK but it wasn't a great deal. That said, I had a good gig in the UK. Worst case, it's not order-of-magnitude different, and besides: proximity to Europe and a culture of realistic holidaying makes up for it (and ... cheeky as it may be to say it ... co-working with people from around the world wins hands down on being stuck in an office 355 days a year with Americans).

You have to work on most weekends and don't get any vacation days?

In the US, 10 days/year is standard. Everywhere else is way more, save perhaps Japan.

I'm in the US, we usually get weekends off as well, which comprise of almost ten days off in a month, let alone a year.