A flight instructor is required to have passed the CPL (or higher exams) - but there is no time limit specified (there are many other threads on this) so a someone who has passed all the exams but missed the 36 month window can still use them for that purpose.
Once you have taken any exams, you have taken them. The validity periods are to do with HOW you want to use them. 2 years for a PPL, 3 years for CPL or IR and 7 years from date of last IR entered in the licence if its for an ATPL.
As exam credits are downward creditable (I used them for my PPL!) That leaves the bizarre situation where at 2½ years they have 'expired' for a PPL but not for a CPL!
Anyway, for the purposes of getting an FI or MP TR - there are no validity periods mentioned. Why do airlines ask for ATPL theory credits? Because it is a requirement for the MP TR.
One thing I've learned is that very few people at the CAA nowadays know very much about the rules. It wouldn't surprise me in the least if getting the IR back gave you another 7 years!
To the OP: These are all ideas and options, use a search function in cap 804, then cross reference to part FCL. If you can spare the money, spend it and you'll qualify much sooner.
Think about this: Every year that you wait, your flying career is a year shorter. That's one less year as a senior Captain. You'll be out of pocket at least £120k in today's money. That's £10k a month. Get on the wagon before the wagon leaves you behind.