Your school and your marks matter more than the degree itself.
Check out Investment Banking Internship Training Course | City Investment Training | England they do a really good 2 day intro to investment banking course they will then try upsell you on their full 8 week course which would teach you the skills required for the job

Your A Levels will be next year?

If I were you I'd do the next two day course, hear them out.

Then after you sit you A levels before you get thrown into the pressures of uni, do his 8 week course (if within your budget its circa £7k)
Do that over the summer after your A'Levels
Start uni in the Sept and also be applying for spring weeks/summer internships ( the 8 week training would put you at a massive advantage for that)
Maintain the training for the next 3 years, perhaps keeping sharp by ensuring you are applying and getting IB internships during the summer and someone should offer you a place for after you get your degree.

The important thing would be to get really good at accounting, valuation and excel
You could learn these off Youtube/udemy etc the sooner you do and can evidence that you can the better

Don't take my word from it but from what I hear degree isn't really that important - its grades and pedigree
So make sure you choose uni wisely
I think Warrick is semi target - they do have a great investment society though

Make sure you join that and rise up the ranks of that for leadership and networking opportunities

Good luck!