I'm taking the same approach I have since mine started nursery....
Enough uniform for a whole week and a bit extra as with working full time, not liking ironing and DH working away all week (so I do taxiing to all activities in the evenings), I really can't be bothered washing and ironing mid week ....
And if I am really lucky DH will iron on Sunday evening 
So for my DD Y7 and DS Y9, I have / will get:
3 x trousers
6 x shirts
2 x jumpers (logo'd and compulsory)
2 x tie (in case they lose it as instant detention to arrive in school without)
Plenty of black socks
1 logo'd sports kit
1 spare sports kit (from tesco or similar)
Trainers
Shoes
Football boots
Apron for crafts etc
Bag (large - no lockers)
Coat (it's grim up north and our teens wear superdry
)
Also get calculator and stationery via school as its cheaper than elsewhere....
But I do an on line order with m&s as soon as their offers start, so 20% off and free delivery and then quidco cash back
HAve spent around £60 on jumpers for both and ties for DD
Around £50 on school logo'd sports kit and around £20 on other sports kit
And £100 ish on m&s uniform
Still to go for shoes, trainers n bags - Dd got superdry coat as Bank had them reduced a couple of weeks back, DS needs bigger one....
Aiming to get it all done on first day both have finished for the summer - then hope they don't grow too much 