When renewing standards are much lower! And that's my fortune as now I would NOT pass an initial but I get through renewal (due to astigmatism). I took initial at the age of 16.
When renewing you do not have dioptric limits anymore. With ageing your miopia can get worse and you will be medical fit as long as your sight remains 6/9 each eye and 6/6 using both eyes with spectacles.
If you check on the CAA medical website you will be able to find all those parameters. Again, if you're borderline now and you want to start an aviation career I think it's wise to take your initial examination as soon as possible.