Off the top of my head I know the pub byStratford Theatre (East London)do a Poetry afternoon on the first or last Sunday of the month - something like that. Call them and find out. Also the Theatre it's selfdoes 'Free' Comedy on Monday evenings.
I used to go to one in Islington - Can't remember the name of the pub - just round the corner from the station - but the venue changed from time to time. Will get back to this thread when I have more info.
This site may be useful.
http://www.zionone.com/listings.asp?...mp;subCatID=52