Really depends on whether or not you plan to both a CPL and IR within 3 years of passing your last exam.
If yes then do the ATPL's as its easier to get the whole lot out of the way at once as much of the CPL theory will appear in the IR syllabus and its better to do it all while its still fresh.
If no then you'd just be wasting your time doing ATPL's, if you've not added an IR to your CPL within 3 years of passing your last exam then you will lose all credits. In the CAA's eyes this is effectively as though you never even sat them!
This would mean that if you then wanted to add an IR at some point later you'd have to redo the I/R exams. Even if you do this however you will still only have a CPL/IR and not a frozen ATPL (ie if you wanted a frozen ATPL you would still have to redo all 14 ATPL exams once again despite having a CPL/IR!!!!)
Either way if you end up only having valid CPL theory you will have to re-sit all 14 ATPL's again to get back to valid ATPL theory and there is indeed no plans for a simple CPL or CPL/IR to ATPL theory upgrade.
With expired or no ATPL theory you are limited to flying only single crew aircraft and would also need to undergo an HPA theory course to fly high performance types. Expired ATPL theory will not even cover you for this I'm afraid.
Therefore for instructing and bush flying in Basic SE/ME Piston types you'd have no problems but it would leave you somewhat restricted should you wish to move on to flying bigger / more complex aircraft should the opportunity arise at some point in the future.....
Best of luck whatever you decide !!
Regards
UA