If the sea level of England rose by, say 2500mm and flooded to 5000mm more Middle England really would move inland to Middle England. Demographically and because Iron Age boats were not adapted for paddling on land most conurbations are on the coast naturally and within a few meters vertically and often horizontally of sandy beaches and tidal inlets. This is why English Indigenous Tribes are drawn to Cleathorpes and Clacton.
Many Councils (without asking anyone if they minded) are drawing up plans for a beseiged England cut off from the world. West Bridgford has walled itself against the Trent river leaving the Proletarian Meadows to flood.
What might become popular to adorn houses may be ceramic sea level cartouches (a cartouche is a wall badge or plaque or title box on a technical drawing)(replacing soppy names like Dunrovin or places mentioned by Lord Byron) built into residential building walls indicating (bragging about) how high one is above sea level. Carpets might be woven to indicate FFL (Finished Floor Level) in better homes. The Prerafaelites’s buidings had cartouches and gold encyptions like packaging has in the latterday.
SUNWAC Flood Plains (Swale, Ure, Aire, Nidd and Calder) would be under water along with most of Lincolnshire, Norfolk and Cambridgeshire. Punting would become a means of getting to college in Cambridge making it cycling and riverboat the main transport method.
Chelsea and Kensington would become the Venice of the South with Artists and Squatters moving in by canoe.
Roman roads would still be above water as they always were skirting the sea round Ely up to Sandringham.
Sea might lap the foothills of The Roughs in Kent, the Military Canal an underwater stream and The Dymchurch Miniature Railway just about on the shoreline. Sheringham railway being on an embankment might still get through. (tbc)