There are indeed many hurdles for the DfT to jump before they take any action against the UK-resident N-reg fleet. Presumably, any rules changes would not just affect N-reg aircraft, but would be a general move against either non-G or, more likely, non-JAR aircraft, even if the N-reg fleet was the intended "victim".
None of this makes it impossible for the DfT to get a consultation and rule-change through the system relatively quickly. Perhaps by as early as the end of this calendar year.
It is no secret that they wish to bring an end to the N-reg fleet, they have said as much quite openly. Whether they will succeed in the face of probable pressure from private owners is another matter.
2D