I don't believe there's any magic that allows Sky Gods some sort of Right to Roam on an airfield. An airfield will generally publish its hours of operation, and what permission you must have (eg PPR by radio / phone / handwritten notification on vellum). Outside hours of operation, almost certainly you don't have permission unless you've made prior arrangements.
If you land without permission, the airport operator may, for example, charge you much higher landing fees and may even, in some circumstances, prevent you taking your plane away until you've paid.
If you land without permission, there is no certainty that the runway is fit for use (for example, sometimes a runway may be used for other purposes) and your insurance may be invalid. If anything goes wrong, the CAA would almost certainly take a dim view.
It's like saying you have a right to park your car at any time in a Sainsbury's car park. If Sainsbury's tell you the car park is closed, then it's closed.
I don't believe an airfield is any different.