pipertommy
Sounds like you have been listening to one of the countless old patronising farts who make up much of the UK GA airport scene and who say things like
"the IMC Rating is a get out of trouble Rating", or
"the IMC Rating is a get into trouble Rating",
"the IMC Rating is only a get out of jail free card,
young man"
etc
etc
etc
Your privileges are IFR in Class D,E,F,G in UK airspace, and that includes all instrument approaches therein, subject to 1800m minimum visibility on the ground, and you can legally fly down to the approach plate minima.
It may be an idea to fly with a half decent instructor, or an experienced IR pilot.
The problem is that a monkey can be taught to fly a plane straight and level in IMC, so saying you won't fly en route IMC doesn't really help you. The instant you enter cloud (which a plain PPL must never do, of course
) you are facing the possibility of having to stay in it for a while, and fly an instrument approach to get back down.
Even flying VMC on top (which is everybody's preference, no matter how good they are, simply because it's so much nicer) means that you may have to fly an approach to get back down.
My suggestion, which is worth what you are paying for it, is that you need to get good enough to fly the common approaches i.e. VOR, NDB/DME, ILS. An ILS is the most important because it is the easiest to fly and will get you down and save your life even in the worst conditions.
FM Immunity is required only for IFR in controlled airspace which for you and in England means Class D. In Class G you are OK. I am not aware of any actual problems ever having been reported (and never met anyone who has) however. It is a theoretical measure to prevent interference from FM radio stations near the top of the FM band e.g. 104MHz, affecting VHF navaids such as ILS and VOR. Anyway, you can fly an ILS into say Biggin but not Bournemouth, well not legally anyway. Of course the schools that fly non-FM-immune planes never go to Class D airports IFR
If you fly an NDB approach then you need a working ADF. There is some debate about this if in Class G but I wouldn't go there. What I would do is use the ADF together with the GPS though, because a 30 degree error on an ADF is common.
Similarly with DME. However any approach which says "DME mandatory" obviously needs one. Sometimes there is a radar range substitute available if you don't have a DME.
I would also carry a very good moving map GPS and spare batteries, with a remote (window mounted) antenna.