What you would end up with is a CPL with CPL theory. If you wanted an IR you would have to do the IR theory (which is fairly easy) and the IR training. However, If you wanted a fATPL you would have to do ALL the exams again (minus VFR comms.). As potkettleblack said, the CAA may allow you to be exept from doing the mandatory in-school revision study.
You could consider doing a single engine IR as that may save you some money in the short term and keep your ATPL valid...Sadly, when you upgrade to a twin you will have to do the full ME/IR test.