Great Yarmouth (Norfolk) is totally flat, a bit down-market though.
Having myself just got back from a day on the Isle of Wight at Cowes Week, I have to add Yarmouth (IOW) which is more up-market, also flat (in the central tourist part), has a harbour, a pier (more of a jetty really), several walks, a castle, only a few p,c,r (it's quite a tiny town centre) and good public transport - buses on the island - and trains at the other end of the island (old Tube stock electric, and heritage steam), car/passenger ferry to Lymington. IOW gets very busy during school holidays.