I have an IR(R) and have been considering options to add EIR or full IR so maybe the notes below will help with some of the apparent confusion above:
IMC Rating, now officially called IR(R): UK ONLY. Can fly in IMC including instrument approaches (with higher minima recommended by CAA than a full IR). Does not give access to Class A. The theory exam is very simple. The IR(R) has been a massive help in my flying in terms of reliability, comfort and safety. I recommend it whole heartedly.
En route IR: EASA qualification. Can fly IMC and allows access to Class A. DOES NOT allow instrument approaches.
IR: EASA qualification, full privileges. Can be passed directly through a traditional course or by the CB IR route. In the CB IR case, training and post-training hours from the IR(R) count which can reduce the cost of the flying element.
The theory for EIR and full EASA IR is the same.
I emailed a number of schools in the UK, some clearly do not know what they are talking about. Most do. The best response I got was from Fly Command at Coventry who do the flying training and recommend the adjacent CTC for the theory.
I reckon the total cost would be about £6000 for me.
To fly Class A you need an aeroplane with BRNAV 5 capability and two altimeters.