My understanding is the derogation is country specific. In other words, if taken up only applies to UK licences in the UK. The use of another licence, say a German one in the UK, I've no idea what happens!
Whole situation is a mess, and I'd not be surprised if some police forces acted a bit strangely. After all, my licence gives a full group B (car) but is valid for Group A (restricted to tricycles) without saying so. A renewed photocard will/should have it, but ask yourself this, a simple licence check has suddenly become very complicated, will the police get it right, will they even bother?
Take another case, two lads, one passed his car test on 18th January, and the other a week later. First lad can drive my Morgan now (if I let him...) but the second one can't and has to wait until he's 21.
It's a mess.
