Music policies at clubs change rapidly... I don't know of any strictly breaks nights in London, for example, but I know they do exist at a handful of clubs, with some regularity.
In Ireland, I had a very hard time finding clubs that weren't house or hip-hop, but then again, I didn't have much of a clubbing budget there.
The best thing to do would be to study some Web sites before your trip (i.e. viewlondon.co.uk, itchylondon.co.uk) to get a good overview of what's out there.
Step 2 - once you get to the UK, at least in London, go to the nearest newsagent and pick up the weekly TimeOut guide, but make sure it's a fresh one. I've been stuck with a week-old one twice because I hadn't looked at the date.
Step 3 - Whatever town you're in, hit a record store or clubby clothing store and check out the flyers. That's gotta be the most fool-proof way to find what you're looking for. On top of that, you can chat up the folks at the store to see which ones are worthwhile.