I was in Paris last year and found Académie De La Bière to be quite enjoyable. Thay had a large selection of Belgian and other international beers along with some good Biere de Garde as well. You also might want to check out La Gueuze which lived up to it's name by having several Lambic beers available.
There is also a Brewpub chain in Paris with a few locations, the original being The Frog & Rosbif (others include The Frog & British Library, The Frog & Princess and The Frog at Bercy Village). The Frog & Rosbif was a raucus place with a younger crowd and more of a British Style Pub decor.
It is just a short train ride to Brussels (approx. 1.5 hrs) and would make a very nice day trip.
In London, my favorite pub is The Black Friar (Be sure to go to the back room which is absolutely amazing to look at). And of course you have to head over to Ye Old Cheshire Cheese if for nothing but the history.