The geographical centre of Britain is the telephone box on Dunsop Bridge Lancashire. The centre of England is Meriden in Warwickshire as other posters have already said, and the middle of Scotland is Forest of Atholl Perthshire.
You are right it is impossible to get further than 150 miles from the sea in England al lthese stories about only 52 miles are absolute rubbish. One example Milton Keynes. Meridian is 149 miles from the nearest coast.
Volvo set up a new distribution depot and decided that the best place was DAVENTRY as they could reach any town in England before their driver was due for a tacho break.
I would doubt that in places such as Penzance and Dover but that is their view.
If Coton in the Elms or wherever is furthest from the Sea, why do they only reference Boston as the nearest seaside point, surely there should be another reference point, say Avon or something..?!?