I know of one definite case who failed not only the Isihara plates at his AME but also the CAA/Gatwick lantern test, and after hassling the CAA for ages got an IMC Rating. Restricted to day flying only. Prior to this happening, it was widely believed that this was impossible.
I know nothing about this bit but would be very suprised if an IR was obtainable, especially from the CAA. The CAA even refuse (repeatedly) an IR if the candidate fails the Class 1 audiogram in one ear, although I've heard of one case who got an IR with one ear failing the initial limits but being in spec for the renewal limits. The fact that a pilot who just passes the renewal limit (in both ears) will be deaf as a post doesn't bother anybody; this is a nice job creation scheme.
Important to note that failing the Isihara test means nothing. It is the lantern test that is definitive. That's what I did; passed the CAA LT 100% while failing the isihara plates miserably. The CAA letter which one gets after that is the most priceless piece of paper - even the FAA accepts it for the FAA PPL (which includes night privileges as standard).
The FAA might offer other options, involving the viewing of actual lamps aimed at the pilot from a control tower. Again, no need to know for me so I have no info. And, one can fly a G-reg, worldwide, on an FAA PPL. The FAA route is THE route of choice if one wants an IR for private flying (or for bizjet flying, perhaps) but then one needs an N-reg plane which basically implies buying one.
Sans Anoraque - I think the bar on night flight just because one can't pass the colour vision tests is a bit stupid. Firstly, most of the people (usually men) that fail the tests can in fact see all the colours in the cockpit just fine. Secondly, tower lights are almost never used these days, and anybody with sense will carry a portable radio in case of a total electrics failure. Thirdly, there is nothing to see when in IMC, so what's the point in see-and-avoid when a suitably rated pilot could be in IMC at any time, in Class G, without any ATS service? It's all barmy.