Best places to go are The End, Fabric, Turnmills (more on Saturday unless u like trancier stuff).
There's also a lot of bars with good DJs round London now, they're free or cheap. Try Medicine in Shoreditch or Plan B in Brixton.
Check out Time Out magazine in any shop they have really good club listings.
But if you're here this Saturday go to Homelands. Dancing from 2pm to 6am to some of the best djs in the world would be good way to spend b'day.