I can only really comment on the FAA guys who were doing there JAR IR at the same time as me. So can't comment on failure rates uk wide ( None of them got first time passes though).
An IR instructor in the UK is very different to a FII in the states system( they need something like 500hrs IFR PIC time for the IRI rating). And the process to allow a FTO to instruct IR's is very costly and intensive. They get well payed though 30k+
The IR in the UK is very rarely done in a single.
Just from the lads I have seen.
The biggest problem seemed to be the fact that the UK uses NDB's alot. And you will have to do a NDB hold and approach as part of your flight test and depending on the examiner both may be simulated single engine. This seemed to cause some problems.
Also the FAA IR's seem to struggle with the RT. There are set ways to do it and it takes abit of retraining to get it correct.
I had this problem as well due to learning in the states.
The only other difference seems to be the method of examination.The CAA has a group of examiners who are employed by them who do the initial IR's. They set and maintian a very high standard. I don't know what their criteria are apart from the set guidelines, but they have descretion over everything. They are the last line of defense before you are released on Jo public and expect to see a professional pilot. I have heard the phrase "he failed before he started the engines" if you turn up not looking the part you will have to be pretty damn hot to get away with it.
As I have never sat with a CPL or IR examiner from the states I don't know how this differs.
We don't do that badly in the Uk with airspace considering the lack of space we have to cram everything into. And in my experence the controllers do try there best to intergrate and allow you to do what you want, but as you have proberly seen on PPrune resources are somewhat stretched. I think the controllers in the US are more GA friendly because more of them fly and its more in the culture of the controllers to accomodate GA But i should imagine a refusal in the uk is more to do with work load than can't be arsed factor
Navigating IFR and VFR seems to be exactly the same in my experence (100+ in the states) apart from we use the ADF alot more but once you get the sod sussed its just as easy as a VOR.
To be honest I think both systems could learn from each other.
Anyone who has actually done this conversion like to comment?
MJ