The licence is valid for life, the ratings need to be kept current to use them. Let's say you go to a school that trains CAA and EASA simultaneously and you get both CPL/IR licences: your first job is likely going to be in the UK. Your licences will start to diverge because you'll only have a CAA type rating. You'll be doing an LPC every year and getting a fresh IR, so you have no worries with the CAA ATPLs, and once you have the requirements and your next LPC rolls around, you'll ask the examiner to do an LST instead, which will give you a full CAA ATPL.
The problem is, as far as your EASA licence is concerned you're still a CPL. Probably with an expired IR. That shouldn't be a problem, because the ATPL exams are valid for 7 years from your last IR - so worst case scenario you can do an IR test once every 7 years to keep them valid. If you ever get a job with an EU airline then they'll give you a type rating which will culminate with an LST and you'll have an EASA ATPL. Alternatively, if you have 500 hours on that type you can skip the course and go straight to the LST.
Thinking about it, *theoretically* if you ONLY got an EASA SEIR the ATPLs exams would technically remain valid for an ATPL for a rolling 7 years. After 36 months you wouldn't be able to use them for an EASA CPL, but you could still use them to convert a foreign ATPL...