ATPL theory and CPL Flight Test must be under the jurisdiction of one JAR member state. However, this does not mean you couldn't do training with another JAR school elsewhere (Spain or Czech Republic...).
If you then want to add a rating, this can be done with any JAR member state school, contrary to popular belief in the UK, the flight test does not have to be with a CAA examiner, but can be with any designated examiner from any JAR country. The CAA will then add the rating to your UK/JAA licence. This is correct for any rating, e. g. multi, IR, type.
Look it up in JAR-FCL - JAR has made it so much more complicated and expensive for us, you might as well play them by their own rules!
Also, you may consider doing an FAA IR first. - You would then only need 15 hours to convert to a JAR IR instead of 55. (Out of which up to 10 could be on a sim!) This is a CAA/UK regulation, non-JAR.
Good luck!