Robin, this may go against EASA's standardisation remit (I do note that you didn't write harmonisation) but there is, under the Basic Regulation, an avenue open to the CAA if an EASA IMCr is rejected which, unfortunately, I believe it will be. At the very least I think it would be negligent of the CAA not to pursue it if the political game fails and no EASA IMCr is agreed upon.
Also, can you shed any light on the Annex II aircraft situation which AIUI, fall outside of the scope of EASA? Presumably, these aircraft will need to be operated on national licences, or will the CAA simply issue them UK-limited EASA licences?
Regards, jez