Yeah we'd be fine on Castle Acre for a while I think. Come to think of it, I didn't see anyone living there when we visited 

There's some high ground in England, but for the real high ground, at least by our standards, you have to go north and up into Scotland. I went up there a few times, it's very beautiful. Great to just drive around looking at all the scenery.

You could always try Wales, Brecon Beacons & Snowdonia.

England has the peak District, the Yorkshire Dales and a few other places 

The Lake District gets up to 900 odd meters (2800 feet). Even if we manage to melt both ice caps, I don't think the sea will get that high.

Don't buy at sea level or near a flood plain. Or anywhere near/in New Orleans.