It's brilliant you are getting this experience at such a young age. It'll be absolutely invaluable when job hunting in the future. Many CS graduates come out of university in the UK ill-prepared to enter the job market and end up taking underpaid junior roles.

But 18k doesn't sound far off the salary I'd expect an inexperienced junior dev to make. Things that might get you a higher salary for this kind of role are a degree or prior experience. If you have prior working experience (even freelance) then you probably are getting a little too low.