@nta16 already gave a load of great advice, and my unsolicited advice is as follows.
(I'll admit, when I was your age, I am unlikely to have followed the advice, but I still think it's good advice. Some of it is unfortunately a bit late, but I'm usually late to threads.)
As a first car, buy the cheapest car to run, ideally with low depreciation, and run it properly with good tyres, good fuel, and good maintenance. Insurance, fuel economy, VED rate, maintenance etc. are all part of running costs.
IMO, do not mod it at all, focus your effort and resource on getting your maintenance absolutely perfect. None of those silly K&N filters, wheel spacers, loud exhausts, tints, wraps, bigger wheels, carbon-fibre bonnets, spoilers, lowering springs etc. etc.
Yes they're all well and good, but IMO they're more trouble than they're worth and usually end up being a costly mistake. (Having made some of those mistakes, I speak from experience.)
If you can't resist modding it, maybe splash out on a better ICE (in car entertainment) system if it needs it, without going mad.
Installing a good dash camera is a sensible mod that may well reward you. (Watch out for it being used as evidence of speeding, though.)
Maintain the car well, use good fuel and run on great tyres. (Great tyres are the best "mod" you'll ever make to a car.)
View that first car as an investment in yourself. You're putting-up with a low powered car you don't find particularly desirable, to allow you get down to a sensible insurance cost on a car you do want.
If you don't mod it and you look after it, with luck you'll get fair money back, and at the least it will be easier to sell than some boy-racer cast-off.
The good news is, you'll find that there's an awful lot of fun to be had in a reliable, well-maintained but low-powered car that's running well on good tyres.
(Dull & quiet cars seem to draw less attention from the cops, too.)
Whatever you do, I'm sure you'll have a ball. But I expect you'll look back on having paid that £6700 insurance with astonishment in a few years.
All that said, it's your money for you to spend as you see fit.
Enjoy and good luck! 👍
( Sorry to sound like a Dad, but I am a Dad, so that's just how I sound. 😄)