Lots of places to go for both!
See what the national portrait gallery are doing, they have great things on often. irving penn a month or two back i loved. Tate modern are running are 'voyeurism' exhibition which may well still be running then, i think there's street/voyeuristic photography featured. and tate modern's a great pace to hang out. Lots of little photo exhibitions are on all the time, time out magazine will have them all. There was Susan Meiselas (sp?) exhibit in a little gallery round the corner from Tate Modern recently, not sure if it's closed. (and the obvious things like the british museum shouldn't be missed if you get the chance!)
Beer... not cheap anywhere. lots of fun places. National Film Theatre on the South bank has a couple of bars, one by the river one more posh at the back, worth going to and seeing people walk past. there's some good old boozers in soho worth checking out, one off old compton street near the palace theatre that i can't remember the name of is really old school. The Lamb and Flag just off Longacre, near Covent Garden is a nice old fashioned pub. If you like cocktails try Floridita, Wardour Street, or the cigar and cocktail bar next to it. Or The Red Fort on Dean Street, opposite the Soho Theatre.
Have a great time.
