Having done the route myself, and know many guys and gals that have also completed this route in under 20 hrs I am baffled as to how this topic keeps coming up.
But there are two things to consider:
1. Do you have any experience of UK airspace prior to starting the IR conversion. If not, go get some. You need practice wearing a high viz vest for starters.
2. Are you trying to do this in difficult airspace. Exeter has only a few limited routes that can be used, so you are most likely to have flown all the routes prior to test (except Cardiff which doesn't allow trainign flights as the controllers cannot deal with more than 1 flight per 5 mins!)
Also, I always here of comparisons but only really consider the following:
How much did it all cost? FAA IR is 40 hrs at US aircraft prices, granting a 40 reduction from the JAA IR. If you then budget 15 hrs in the multi aircraft in the UK, you will still come up way less than the full 55 hr course in an aircraft in the UK.
Compare apples with apples etc. Maybe consider Spain as an alternative too. Much cheaper there. As we all know these days, standards are not the CAA's objective. Its taking your money thats important to them.
Also, if the 170 was signed, how is it the fault of the FAA IR system, when you fail a checkride?
Simple - It isn't!