I don't know anything about buying land, bit I do know PROPERTY IS THEFT. Fact.

;-)

Farmland sells for as little as 10p a square metre
In ordinary towns, development land sells for £ hundreds/sqm

Not much help I suppose. You pay whatever its worth to you if the landowner will sell it to you.

Look for land being sold at auction, you might get a bargain.

A small piece of land is unlikely to cost enough to attract stamp duty land tax (unless you're buying a parking space in London!):

https://www.gov.uk/stamp-duty-land-tax-rates

You will need to find out if the land is registered or not:

http://www.landregistry.gov.uk/public/property-ownership

You will need to make sure that you have the access you need, if the land doesn't adjoin a public highway.

You should also check out things like flood risk:

http://maps.environment-agency.gov.uk/wiyby/wiybyController?x=357683.0&y=355134.0&scale=1&layerGroups=default&ep=map&textonly=off&lang=_e&topic=floodmap

And remember, if there's anything prehistoric on it, we will all want to come and visit!