The IR(R) doesn't exist, it is just an administrative fix, devised by the UK CAA, to to allow the IMC Rating to be entered into an EASA licence. The IMC Rating will continue unchanged, whatever happens in April; you just might not be able to enter it into an EASA licence. However, you can always obtain a UK PPL(A) alongside the EASA one and keep the Rating current in that.
Of course, this may all be overtaken by events on 29th March when, if we leave the EU on WTO rules, all UK-issued EASA licences will immediately become UK licences and the IMC Rating can be entered as such.