The other factor is that while the UK has no tax on avtur, and there is as yet no sign of one coming in (despite being used by countless turbine helicopters for decades) the stuff is taxed as avgas is some other places in Europe. I've heard UNCONFIRMED reports of other European countries doing this, unless an AOC is produced.
Yet another factor is that (and this I know first hand) one can get tax free avgas on production of an AOC in quite a number of places, outside the UK. I've been offered this in Greece and Italy and, I believe, France. Spain seems to forget the tax anyway if you flash a Ltd Co Air BP fuel card.
The fuel cost saving makes it worth getting an AOC even for a small flight training operation. Obviously the AOC doesn't have to relate to the school's activity in any way
You just need the piece of paper.
All this reduces the already quite small European market for diesel engines.