Property in England is sold with the title registered at the Land Registry, so you don't have deeds but a certificate of title which defines where it is and lists all obligations which go with it (rights of way, easements, mortgages etc). That's the doc you need to see, and which a solicitor can get ,before any surveyor gets involved.
In East Anglia prime agricultural land hit £10k an acre a couple of years ago. Land with resi pp is awesomely expensive in the posher bits. Take the price of the house(s) you will build, deduct the construction cost, deduct a profit margin for the developer and the residual is the land value. When you can get four £1m houses to the acre, even spending several hundred thousand on each house leaves land a mite pricey.
The South-west is a lot cheaper, but small parcels of land sell at a big premium to agricultural land value because an awful lot of people want a bit of land which they then don't know what to do with. That's why we farm five of our neighbours' "bits of land"
Check the title and walk the land, especially the boundarys and the access.