Do the IR in Spain, and save yourself a packet of money. The instructors (at least, the ones I've met) are every bit as professional as the UK ones. Many are ex-Iberia or other airline professionals. As others have pointed out, once the CPL/IR is on your licence, it's a JAR document, issued by the CAA, and doesn't say in which JAR country the qualification was obtained. I'm sure most airlines wouldn't care either. Despite the hype, IMHO training at Oxford or Cabair isn't the be-all-and-end-all of professional training.
Why should you pay the CAA £720 (or whatever silly amount it is now) just for the IRT when you can do the same test for a fraction of the amount in Spain, just to finance the CAA's fat margins?
Have you tried AirPal at Palma? They fly BE76's, have a good reputation and very competitive prices. Palma in winter/spring would be a great place to train and you would get some interesting weather, not as bad as the UK but enough to make for some challenging flying.
Good luck