Hey, I hardly ever post!
CAP 168 ‘Licensing of Aerodromes’ will tell you more than you want to know, but it’s to do with the CAA having inspected the runway and being satisfied that it meets various criteria. One of the things they check is making sure that there are no obstacles in the approach/climb-out areas. For instance, at Little Gransden we have one licensed runway (that includes an unlicensed starter extension at the eastern end! — effectively the threshold is displaced due to a road and a hedge at the eastern boundary), and two unlicensed runways that have large trees or power lines in the way. I’m sure there are more things they look at too.
If you’re training for a PPL, you’re not allowed to operate into/out of unlicensed airfields — certainly if you’re solo, and possibly even if dual. There are certain exemptions for microlights if I remember rightly.