I think the issue is that they are not giving you the IR(R) because your IR is MP and they have forgotten about the inbuilt privileges.
The table of conversion criteria in CAP804 Section 4 Part P page 31 states that to instruct for the IR(R) the holder must:
Hold a valid FI(A) and a valid IMC
rating prior to 8 April 2014; or a
UK CPL(A)/ATPL(A) issued prior
to 1 July 2000.
Therefore they should issue the rating. They issued mine but, I do hold a SP IR as well.