I agree with Harves.
If instructing is what you want to do, then do your CPL, then FI. If instructing isn't necessarilly what you want to do, then do your CPL, then IR - or, alternatively, your IR, then your CPL.
Remember that the minimum entry requirements for an FI course are higher if you don't have a CPL (150hrs P1 instead of 100). And that although, in theory, you can work as an unpaid FI without a CPL, in practice you won't find many people willing to offer you a job, and you won't be able to work for yourself because your FI rating will be restricted initially. So there's unlikely to be any benefit from doing it first.
FFF
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