I am not allowed to train anybody in instrument flying outside an official course.
Which is also quite incorrect and as a JAR IR Instructor I am a little shocked that you know so little of the rules.
A JAR Instructor without Instrument restrictions can teach Instrument flying to a pilot without being part of any official course. It is only of the training is towards the issue of a rating where certain criteria have to be met. The holder of an NPPL may approach any unrestricted Instructor who is able to teach instrument flying and be taught the entire content of the IMCr or even IR without being attached to any training organisation. There is of course no way to attach the rating to an NPPL but it would certainly enhance the pilots skills and improve their awareness of IFR flying and planning.
Why can't someone rock up to a FI and say "Look, I have an AI, a TC and a VOR CDI thing, can you teach me to use them please?....oh and while you are at it could you please teach me to fly an autopilot coupled ILS please"....(that last bit might be a problem in Europe as I doubt many training organisations have a funtioning AP )....
They can.