For serious beer geekage I'd recommend The Rake in Borough, Craft Beer Co. in Clerkenwell or Islington and the Euston Tap at Euston. For more traditional offerings you've the likes of The Harp in Covent Garden, The Jerusalem Tavern in Clerkenwell, owned by St Peter's, and The Royal Oak in Borough which is Harvey's London pub. But London has so much to offer it's a lot easier to start with where you're staying and what you'll be doing, and then build a pub itinerary on that.