Because of the increasing number of runway infringements in the past few years (38 alone at a major twin runway airport somewhere west of London) there is a big push going on from the CAA and NATS to make people more aware of them....
Hence...
If you land without clearance, lost or not, you have committed a runway infringement and it will be very closely looked at. Obviously, if you have a technical or radio problem then the circumstances will be examined. If you have landed at an airport without clearance because you are lost then you can expect some quite heavy penalties and a thorough investigation into your skills and competency as a private pilot