If I screw up the Lantern Test at Gatwick, is that not the end of the line
It is the end of line for a JAA IR, I believe. As is having out of spec hearing in one ear. I do know of somebody who got a CAA IMC Rating despite failing the colour vis tests, with a daylight only restriction. This is apparently very rare but is within the authority of the CAA as the IFR privileges are limited to the UK anyway.
But if you can get an FAA medical then you have an FAA medical! An FAA medical (1,2, or 3) meets ICAO and if you fly an N-reg plane you are done.
The Irish have reportedly decided that the FAA C3 med doesn't meet ICAO. They are almost certainly wrong but in Ireland anything goes and it's not worth your hassle; just go for the Class 2.
I have both JAA and FAA medicals and the FAA C2 is actually slightly more strict on things that matter than the JAA C2, but one's eyesight is never the same on any two occassions anyway. No good going for an eye test if you are very tired and have been doing a lot of reading, for example.
Go for it. Your colour vision is tested only once in your life, I was told.
DRJAD - the IMC Confuser is a great book. If one can work through all the examples in that, one must have a good understanding of the subject. The CAA multiple choice questions are full of tricks, with some of the computed headings differing by just 1 degree - to check you are using the slide rule correctly.