There is conflicting data on what the CAA use to decide.
A well known lawyer has often said they go after people seemingly randomly. This may be true but from the published details (nearly all being
successful cases) they either rarely do it, or they bungle most of them.
The CAA themselves (and I have spoken to their head of enforcement a while ago, on a different but related subject) claim they prosecute only the most serious cases. Again, the published data either supports this, or they prosecute loads and bungle most of them...
Personally I think the latter is the case (they rarely prosecute). According to the Ontrack survey, there are several hundred serious CAS busts every year; big ones where jet transport traffic is stopped from departing / diverted / etc. I don't know how many get away but clearly only a tiny percentage get done for it.
I would guess that most of these would be
equally serious, in that none will be deliberate, all will be navigation errors, and the main variable will be the degree to which the pilot contributed to the error. Speculating... not carrying charts is going to make you look a right d1ck.
Rumour has it that the CAA go especially after people with a bit of an attitude, or history of winding them up. This is exactly what the police has always done, and it makes sense since the CAA dept is made up of former police officers.
There will always be CAS busts - the PPL training procedure is too useless for today's airspace.
AFAIK nobody ever has a word with the school or the instructor. This climate makes it easy for a decent lawyer to get you off the hook for most things.