Barney_Gumble
There are several thousand employees at the CAA and they have to paid somehow. Then you have all the licensed engineers, doctors, parts distributors who charge hundreds or thousands of pounds markup for rubber-stamping a certificate, you name it, who depend on the "system" for their living too. This is why there are various restrictions many of which appear to be bizzare
Otherwise, every plane flying in the UK would be perfectly legally registered in Liberia where the maintenance regime and license / medical issues are ... shall we say ... more "liberal"
I am pretty damn sure that plenty of people have been trained over the past years in Private CofA planes. I have no idea but I don't suppose it would invalidate their logged hours so long as the student participated in good faith - let's face it, how many students know this stuff? It's only when you become an owner that you have to know it; even many syndicate members don't know it. I was offered IMC Rating training in a Private CofA plane once, by a very experienced instructor who most definitely "knew" (I discussed it with him at length).
You've got to be very careful what you buy (whole or share) if you plan to do additional training. It's not only the CofA type but also the equipment fitted, for example if an ADF does not work then you might just about be able to legally train (in VMC with an instructor) but you won't be able to do many/most IAPs. And a group keen to get rid of a troublesome member isn't going to explain this to you!