You do not need landowner's permission to land in the UK, subject to the requirements of Rule 5. (If landing near an event or in a congested area).
However, if you land somewhere and the owner or owner's representative asks you to leave, you must go within 3 "asks" or you will have committed a breach of an 18th century trespass law.
If you damage anything that the owner/freeholder has put on the land, you will be responsible for the damage.
The definition of freehold is the right to fence a piece of land. Ultimately, the Crown owns all land in the UK. It's what you put on the land (crops, buildings etc) that is your property.