It's true - a UK CAA NATIONAL CPL that does not have the letters JAA stamped on the front has inbuilt IMC privileges. The JAA does not.
As to "converting" the IR-SPA-ME (multi I/R) to IR-SPA-SE (SEP I/R), it's all the same creature - the multi is valid on the single, but not vice versa.
If you have to renew/revalidate in an aircraft then you are looking at just the price difference between an MEP rental and an SEP rental. £100? $100 (about £60?) if you go with your USA option - though no landing fees or aproach fees.
If your I/R is still current, perhaps you could apply for an IMC rating, through the usual channels of the CAA, and have that issued for 25 months. When EASA comes along and kills it off, then renew the I/R to full strength?
You have six years from date of last flight test to renew your UK issued JAA I/R without any additional hassles....so just renew sporadically.