This is being discussed currently on Rotorheads! The rules are different for fixed wing.
It's 3 years BTW, not 5 in which to get an IR after ATPL(H) passes.
However, an ATPL is really only needed for North Sea ops where it's a two pilot operation. Onshore work usually just requires CPL(H) theory and then, if you want to get an IR, you have another 7 exams. You will then have a CPL/IR which, after recent rule changes can no longer be upgraded to an ATPL(H) - it used to be!
If you just plan on instructing for example, the CPL exams would be sufficient. But, if I was you, now, I would do the ATPL exams. Yes, you may have to resit them if you don't get an IR in 3 years but it's easier to resit something you've already studied once!
I've nearly finished the CPL exams; given the rule change, I wished I'd sat ATPLs.
Hope that helps.
Cheers
Whirls