The sea level in Farnsfield is the same as everywhere else in the UK. The Ordnance Survey have fixed their datum in Cornwall. Here in the U.K. we define heights above sea level in terms of 'Ordnance Datum, Newlyn'
This will only change if 'Global Warming' turns out to be real and sea levels start to rise. You should be safe for a few years yet though - but don't make any long-term investments in property in the area.