Re FM immunity, this is worth checking but my understanding is this:
It is mandatory for IFR
in controlled airspace. This means Class A-E.
Most planes available for self fly hire are
not FM immune, which is why PPL-level instrument training usually takes place under VFR and keeping out of CAS, and if they go into CAS, in IMC.... well.... it's a case of ask no questions tell no lies
So I think that flying the typical old wreck, you could fly an ILS into Biggin (VFR or IFR) but not into Bournemouth. Into the latter, you would have to do it as a practice approach under VFR conditions.
This is a big reason why the GNS430 is popular: you get an FM immune radio, VOR and LOC. People certainly aren't buying it for it's superb large legible moving map display
To replace a couple of conventional radio/nav receivers with FM immune ones is a few k, which is why nobody does it unless they have to.
Unfortunately, once you start looking up the mandatory equipment requirements for IFR in CAS, you find you need an ADF too... and a DME.
On the IMCR minima, I agree with giloc; it's a recommendation. It says so.