Whitstable has a nice hotel, The Grand, and the Oyster House rents out adorable converted fishermen's huts on a B & B basis.
If you feel the need for quiet space to be by the sea there might be something suitable in Orford/Dunwich/Southwold? The Suffolk and North Essex coasts are so beautiful.
Suffolk is lovely if you can find a good b&b .. there's a fabulous farm park (Easton Farm Park) an otter and deer sanctuary, a safari park with a little train that takes you on a tour, or you can walk round plus loads of beaches (either pebbles or mixed)
its about 2.5 hours from SE london (straight up A12)