The simplest way to ensure that your land is not acquired by someone else through adverse possession is to make an application to be 'registered as a person to be notified of a squatter’s application', using form ADV2:
http://www.landregistry.gov.uk/_media/downloads/forms/ADV2.pdf
The fee for doing so appears to be £40. (The Land Registry could advise you about the fee anyway).
You would then be automatically notified if anyone tried to claim the land through adverse possession, allowing you to formally object (which would be all that would be required to block the claim).
The foregoing assumes that the land is actually registered, with you as the title holder. If not, your first step should be to seek registration of the land.