They could shut down trusts but they can't stop you, if for example you know (and trust) somebody who is a US citizen, giving them the aircraft on paper.
The only difference is that this trusted person could run off with it. So.... somebody needs to pay for an indemnity cover. Which takes us back to the Delaware Trust situation, more or less.
The only way the USA could clobber things would be by insisting the aircraft spends a minimum time each year in the USA. This is already an issue with planes owned by US corporate bodies, I believe - though in reality there are no checks and unlike with G-reg there is no reliable database where the planes actually are sitting.
There are plane spotter websites but they are hardly going to be complete. However they could be used to compile a partial list of UK resident N-reg planes and I gather that the CAA already does exactly this.
We must be grateful for the existing privileges, in a world where practically every aspect of freedom is continually tightening up.