I'll third a Timbuk2 if they do one the right size. I have had one for several years and, as you would imagine, it was excellent for taking crap to university on my bike. Five or so years later I am still taking it to work every day and all the pockets are very useful.

I have carried god knows what in mine in *all* weathers and it refuses to break. The only thing that has gone is the velcro strips on the flap but I think I can glue some new stuff on considering the rubbery nature of the lining (which is another good thing - it's *very* easy to clean out squashed melted chocolate)

Edit: Bum-bags/fanny packs (tee hee) are useful but deeply unfashionable and would probably make you an instant mugging target in London due to the loser/tourist image.

Gareth