Open mics I like in London:

Tuesdays: Needle & Thread open mic at the Herne Hill Half Moon (
Tuesdays: The Cavendish Arms in Stockwell/Vauxhall
Wednesday: Easycome Acoustic at the Old Nuns Head (though not exactly an open mic and a bit shambolic. some great regulars though)
Thursday: Lantern Society at the Betsey Trotwood in Farringdon (fortnightly)
Thursday: Bat Country at the Hideaway Bar in Tufnell Park (once a month)

excellent listings source for open mic info up and down the UK:

www.thevac.co.uk