Under the CAA/EASA rules, a powered aircraft can fly IMC without a Xpdr and without talking to a radar service.
Don't be so sure. I have not read the 600+ page document describing the EASA proposal for FCL but from the summary I understand that both the IMC rating (for power) and the Cloud Flying rating (for gliders) will go away.
And I would not be surprised if further European harmonization will eventually mean that flight in IMC requires an IFR flight plan, an IR rating, controlled airspace and an IFR capable aircraft. Including a transponder. This is already the case (or, as far as mode S is concerned, happening soon) on mainland Europe. Why would the UK not follow suit under EASA, which is after all a European organization?