Salaries in the UK are lower than the Bay Area, but I wouldn't say they're "horrendously low". I can see why that ad would make you think that, though: it really is absurdly low for London, but I don't think it's representative.

Sadly, in Dickensian Britain, a .Net developer is worth the princely sum of £1 a day, while a Python/Django expert can at best hope for £600 a year!

http://i.imgur.com/3Aw8oop.png (genuine screenshot from the Independent, a British newspaper)