Yeah, the specifics are definitely a little vague when it comes to things like buildings and vehicles.
This page does get into a little more detail and clears up some of the ambiguity present in the simpler poster, including some examples of what would constitute a built-up area. Note also that you can get closer than 50m if you have the permission of the owner to do so (e.g. to survey a roof for storm damage).
My personal take is that a small hamlet would be OK for 50m, as would a farm consisting of several outbuildings, but if it is undeniably a village (e.g. it's signed as a village, has a church, or some form of retail) then assume it's in the "built-up" category and use the 150m rule. Obviously, if anything goes seriously awry then all bets are off and the only opinion that is going to matter will be that of the court though, so always better to err on the side of caution whenever you can.