I've been to Bolton, in fact my family come from there. Gastank, I'd say that you shouldn't hold up your hopes. It's famous for the being the cradle of the Industrial Revolution in Victorian times, years of neglect, and the upsurgent home of the BNP. (Or is that Oldham?).
The only two things I remember about Henley are that they don't do Tri when it's peak season, because they're rowing. And the race reports from the tri had all the racers hanging onto the pontoon trying not to get swept down the Thames at the start.
But what do I know?
Personally, I'm eyeballing IM Wales. 2013.