You didn't say what year she is in, if she is in year 7 and planning to do A levels, that is a long time for a laptop to last
Having bought hundreds of laptops (worked in IT in schools) since 2002, the only make I would recommend and the only make I will buy for myself and family now is HP.
My daughter works for the local university, and I see that her work supplied laptop is also HP.
She should get MS Office through the school, so that should save a bit.
Some things to look out for generally.
I would suggest a 15" screen, it makes a big difference to eye strain prevention, and definitely if she wears glasses.
It seems that most new laptops are using M2 SSDs as their primary drive, and most come with a 256GB drive, which is not going to be big enough. (Unless she only uses it for school work without many photos)
Some laptops have provision to fit another internal drive (or remove the DVD drive and fit an SSD in its place) otherwise you will soon need to upgrade the M2 SSD, which means cloning or reinstalling everything if done after purchase. Presently Intel 1TB M2 drives are only about £70 (just done two of mine) so would be cheaper to do at the time of purchase.
Consider AMD Ryzen and Radeon graphics, I have found AMD very good all round and better value.
As you want it to last a long time, I would say 16GB memory, at least be able to upgrade to that (some makes have the memory soldered to the main board and can't be upgraded)
A USB C port, and 3 USB ports
External monitor connection (either HDMI or one that a cheap HDMI adapter can be used)
Built in card reader if she is doing photography (either at school or home)
At least DirectX 11 compatibility
Decent battery life
Full size keyboard with numeric keypad
If she will be logging on to the school network or VPN, check with the school what windows version she will need (Home version may not work)
If the school uses Smartwater, ask if they will mark and register her laptop (and put a small sticker on it. I put the stickers on the lid so that it can be seen easily)
If she needs to maintain competitive performance, maybe 4 years is as long as a good one will still be good, I find at home that 6 years old is starting to make a new one essential
I don't know if you will do it for £500, I would guess it will be more like £600 without making compromises, and £400 with more compromises, which may not show at first, but are sure to in a year or twos time.
It is also worth asking the school if they have any suggestions and if they can get you a more attractive price.