O2, £15 a month for their top package.
Which is up to 20mb.

You do need a BT landline though.
( Noit happy baby orangutan, Tue 26 Aug 2008, 14:37, archived)

This,
or Be, who run the service for O2. Great value, we had Be in my shared Uni house a couple of years back, very very quick.
( mr_wicksy is.... old., Tue 26 Aug 2008, 14:42, archived)