Brewpubs in London:
The Orange (Watney's), London
Porterhouse, Soho, London
Zerodegrees, trendy brew-bar pizza restaurant in Blackheath village, the trendiest spot in the otherwise run down area of South East London

Plough Inn, Banbury
Royal Clarence, Somerset
Holt, Plant & Deakin (Allied Breweries), Birmingham
Frog & Frigate, Southampton
Miller's Thumb Brewing Company, at The Canal, Glasgow, Scotland

(from Oakes)
Two mandatory stops - White Horse on Parson's Green and the Wenlock Arms, which is nearby to the Pitfield Beer Shop in Shoreditch. In Whitechapel, there is Bottles, another beer store with a huge selection. Unless you just won the lotto, don't even think about shipping beers back with you.

Best brewpubs for beer: O'Hanlon's in Clerkenwell and Porterhouse in Covent Garden

East London & City Beer Guide:

Beerseller of SilkTork: The Bitter End at Bromley

I have just been to London a month ago and found some interesting beers at a Safeway such as Brooklyn Chocolate Stout, Dogfish Head Raison D Etre, Goose Island and other American beers. The only thing is that I went to a couple of different Safeways and some of them had a nice selection while others only had crap.

Selfridges or in Safeway or Tesco or Somerfield or Waitrose

Beershop: Bottles (London)