From the PPL/IR website,
Q: Will applications for UK IMC Ratings to be included in existing JAR-FCL pilot licences made between 1 July 2012 and 8 Apr 2014 result in the holder's licence being converted to a Part-FCL licence with IR(Restricted)?
A: Yes.
So you have until the 8th April 2014, after which time you can only exercise the privileges on EASA aircraft. Although nobody seems to know if you can add a IR(R) directly to a Part-FCL. But as long as you have a JAA licence, you can add the IMCr to that, then convert it when it expires.
And from the horse's mouth, the CAA interim statement on the IMCr:
On conversion to a Part-FCL aeroplane licence, the holder of a UK-issued aeroplane licence that includes the privileges of the UK IMCR prior to 8th April 2014 will have a Restricted IR included in the Part-FCL licence.
So provided you have it on your JAA licence prior to conversion, or are converting to Part-FCL when you apply to get the rating added, or leave it on your JAA licence and convert later, it shouldn't be an issue, and there's no need for a supplementary UK licence (until after 2014 where you'll need it to fly non-EASA types anyway).
See
CAA Interim Statement on the IMCr
Certainly, my IMCr instructor sees no problem getting the rating added after the 17th Sept.