So we agree the CAA could clobber UK resident N-reg in various ways. The question is whether they will ever bother.
The biggest driver in the CAA is probably money (even if they understandably must dress up any regulations in more paletable terms). I am sure that N-reg in UK GA is only a tiny part of the UK N-reg scene, seen in the context of CofA revenue loss to the CAA. For any training or commercial work, it has to be a G-reg, so the great majority of the UK GA fleet will never go N-reg. The sole owner / syndicate fleet which could ever go N-reg is very small; private owners have the Private CofA option which negates most maintenance regime savings and a miniscule % of those have an IR. And very few syndicates have enough "IR" members to make N-reg worth doing.
As in the UK we have the IMC Rating, the real driver behind the FAA IR is European flying, and that pretty well means airways. The % of UK PPLs that have an IR (of any sort) is tiny and always will be very small, not least because most of the UK fleet is not airways legal.
I don't think it is easy to get a plane which is equipped well enough for this, for much under £100k, and the vast majority of the UK fleet is nowhere near that figure.
Also much of what the CAA could do would be ineffective unless the whole of the EU did the same. Otherwise, multi-aircraft N-reg operators could get around any residence restriction by rotating planes through the UK.
There are also higher political considerations. The UK is a friend of the USA so they won't do some sort of blatent "up yours" gesture which perhaps the French DGAC might be able to do.
So perhaps we are making a bigger meal of it than is called for - although discussing it is certainly a good idea. Those who need to go N-reg will do so anyway but they will always be very few in number.