OK, but I've no axe to grind, as far as I'm concerned you can do your IR where you want.
I don't think your argument is right, though. It works for Heathrow because the R/T and procedures into and out of big airports are fairly international and usually pretty straightforward. That makes the argument sound good but it's less convincing when you consider small regional airports and the more difficult and ritualised aspects of the IR test. I would be prepared to bet that most FAA ATPL holders would fail a JAA IR test if they had to do it without any preparation, in a Duchess, flying out of somewhere like Exeter or Bournemouth.
Before I get jumped on, I also doubt a UK ATPL holder could pass an American IR test without being properly prepared in theatre. Apart from the flying, the oral exam would blow his/her socks off.