Ok, in that case I recommend:
The Good Ship, Kilburn http://www.thegoodship.co.uk/
Very nice venue, band nights several times a week, my mate got booked within about 3 days of me suggesting it to him. Incidentally, the jukebox is free.
Babylou, Brixton http://www.babalou.net/
Nice North African themed place that does a band night on Fridays (at least).
The Fiddlers Elbow, Camden http://www.thefiddlerselbow.co.uk/
Looks as scummy as hell but is actually ok. They have a bands wanted ad on their website!
The Underbelly, Hoxton http://www.underbellyhoxton.com/
Ok, it's Hoxton-y but it's quite fun.
The Betsey Trotwood, Clerkenwell http://www.thebetsey.com/
Went for a party rather than a gig, but they have regular nights. Also they have Scroob on the website
And finally, avoid The Windmill in Brixton like the plague. You're not going to get mugged but it's a nasty concrete box with crap acoustics.
Will that do ya? Incidentally make sure you let us know if you're playing, I'll turn up to the opening of an envelope so you're guaranteed at least one person who'll come and watch you.