I would buy a blazer that fits and is not too big to be honest as they don't last that long.
If you buy a blazer with Year 8 in mind, then by Year 8 it will likely be ripped or too worn.
Funnily enough, my son hasn't grown much at all this year anyway - although he eats more than me! Maybe all the calories were needed to complete the homework
My son's blazer was in rags by Easter, rips inside the lining, seams coming undone everywhere - I have no idea what he has been doing with it! It is ready for recycling as scrap now as in the last few weeks of term the seams on the side came away completely. What I really don't like is having to sew the badge on yourself (as I have never been able to sew - it is a slow and arduous process!). I initially paid Early Years to sew it on, but the stitching came undone after a few months, and since then, I've had to repeatedly sew it back on myself.
I buy my son's trousers from Next and they have been really good quality, no wear or holes on them thankfully!
One thing I did buy a little big was the PE and rugby kit - they're very expensive, and the name has to be sewn on each item, too. They are still in perfect condition though and will fit throughout year 8, I would imagine.