The 3 month window is not a UK only thing. JAR-FCL provides that for a rating revalidated within the last 3months of validity the next period starts from the expiry date of the rating and not from the date the revalidation was completed on........i.e. do the necessary a bit early and you don't loose out.
Never used to be like that though. In the past I have had mine revalidated from month 13 for a further 2 years from the original date of expiry. Then it all went to rat !!!!! and we're stuck with a stupid system which only seems to be in place to complicate things.
It is ok Bose saying "do it early", well for my last reval although although I had plenty of time logged I wanted to do it in the last 3 months understandably. I had the time logged but not the 1 hr with instructor. And to kill two birds with one stone I did the reval with a FE acting as FI - he then signed my paperwork at the same time, saving me two 80 mile round trips.
For 2 of the last 3 months I was away, and for the remaining 1 the weather was attrocious. I made it with 2 days to spare, only because the PPL student cancelled as it was 30kt winds.
I can't see why if you have the experience, then you can't reval at a) any time in the last 12 months or b) any time AFTER the date of expiry, but from the date of expiry you are not allowed to fly, and if done AFTER expiry then it runs from the original expiry date.
Even better, lets just have a BFR and be done with all this nonsense of mixing FIs and examiners....