As others have mentioned, there are a lot of variables. Many you won't know now, but some of it you can influence.
My younger two children are the same age was one of my colleague's two children. She goes on about girls are so expensive with the hair products, make up, and shopping trips she does with her daughters. That's entirely because my colleague likes doing that with her daughter, not because all teenage girls are like that. My colleague encourages her girls into expensive brands, hobbies and trips - she gave her kid for a post-GCSE gift was a two week trip in the Americas. On results day, mine got a cookie cake from Millie's and new steel-toe cap boots to meet the requirements of the Level 3 course she got into. They're worlds apart, and the expenses show.
I found clothes hardest between about 9ish and nearly adult size, there is far less secondhand and even lower price stores seem to go out of those sizes faster than the younger sizes plus growth spurts can ruin best laid plans. Having a secondary school with a strong secondhand options can help a lot for blazers and jumpers. Once adult size, their clothes are lasting longer and there are more options across more price ranges.
As for uni - there are a lot of options, and there maybe more when your child gets to be that age. My oldest spent a year working between college and uni, went the cadetship route, similar to a degree apprenticeship, so tuition and monthly bursary paid so even though the required halls for the first year ate most of that bursary, our only expense was his phone and visiting him. I know a lot of young people taking on additional Level 3 qualifications or taking council courses that lead to direct employment rather than going the traditional uni route.