In any event, I think the BIR is not a replacement for the IMC rating. The BIR significantly more demanding than the IMCR. I think therefore the IMCR should be retained as an entry level IFR qualification.
There can be no justification for the UK to end the IMCr given its fantastic safety record. The CAA have been immensely proud of the rating and fought and cajoled EASA for many years for the EU to adopt it. It will be strange to let it go now they are not forced to. My money is that it will be retained even if the BIR adopted. As for any agreements being achieved with the EU (EASA) for anything at all, this is going to be difficult for some time, the atmosphere within the powers of Europe is to make life for the UK difficult whenever they can.