The thing about London is there is sooooooooooo much to do so you're basically spoilt for choice and it all depends on what you're into. Barbie's wkends are something like this..............
Fridays there's nowhere really that I have to go to now, used to go to Turnmills all the time but haven't been there on a Friday night for a year now but if you like trancey/proggy stuff I think it's still rammed. Pacha London has unfortunately given over it's Friday nights to School Disco

which is a damn shame cos they had some brilliant line-ups over the summer - Erick, Morales, Richard F................
Saturdays I might go to Turnmills esp for City Loud, that's the 3rd Saturday of each month but Metro Groove and Roach are also good.
Also Pacha London has really sorted it's act out now on Saturday nights and have got some decent line-ups now.
The End is always a good night, look out for Darren Emerson's Underwater nights.
The Cross is also pretty good, really nice club and dressed up crowd, check the listings as it's different every wk - one wk is Renaissance, one wk Serious, one Type and I can't remember the other.
Sometimes I'll even venture to Ministry if the line-up is right, Louie Vega plays down there down now and again, the next time he's there is the 23rd November and his 5/6 hour sets there are legendary.
It basically depends what you're into, all my suggestions are more on a funky tip but whatever you're into I'm sure you'll find it in London.