does the aircraft equipment also have to be IFR immune
I think what you are refering to is "FM immune". This applies to the radio and VOR/Glideslope equipment, but not to the ADF, which is not FM. This is a rather crazy ICAO standard based on the belief that FM equipment can be affected by other signals and so produce misleading readings. The fact that ADF is subject to many more influences does not seem to prevent the almost religeous attachment to NDB approaches over GPS
Many aircraft have FM immune equipment and I don't think there is any additional cost of maintaining them over non-FM immune. Not having FM immune means that you cannot do an ILS or VOR approach in IMC (but you can legally do the far less accurate NDB/DME approach).
Of far more importance is the lack of de icing or anti icing on most single engined GA aircraft. This severely limits flight in IMC for substantial parts of the year. So for that matter does not having an auto pilot as prolonged flight in IMC is very taxing without one.
Unless you plan prolongued tours or business trips in Europe IFR and have the right sort of aircraft then an IR is an expensive luxury. As has already been said, you can fly IFR in all UK airspace except class A with an IMC. Personally, I would not plan a flight which involved any significant IMC on the back of an IMC rating - flying IFR does not of course mean that you are in IMC.