Pubs in the UK don't really set themselves up as craft brew places, because almost all of them sell real ale (if they don't, leave) and it's just a question of where they get their selection from. It's fair to say that all
the real ale served in the pubs is stuff you can't get in the US, just by being cask ale. I suggest you should focus on finding good pubs, period. The best for beer are 'open houses' where the landlord is not tied to one brewery and can sell what he likes. These days many 'tied' houses also carry one or two guest beers, so you should find interesting stuff without trying too hard. Your best bet is to check out websites like www.fancyapint.com
(my preferred site, particularly useful for the search by tube station, and because the rating system is heavily weighted to the quality/range of beer), and www.beerintheevening.com
. This page is a good start - http://www.fancyapint.com/more/best-pubs-for/beer.html
Also, depending on where you are staying, the Fuller's brewery really isn't far from central London - only a shortish tube ride and a ten minute walk. Tie in a stroll along the river to the Dove (if it's a nice day) and you're all set. Have a great time!