Why not buy a copy of LASORS? It will answer such questions for you.
It's a Night Qualification, not a Night Rating. The IMC Rating is only valid in UK airpsace.
Why would you want to obtain a PPL/IR unless you're a wealthy private owner? Professionally, it's probable that you would go from PPL to IMC to CPL and/or night......and then the IR. Perhaps obtaining a MEP Class Rating at the same time as the IR.