Hi Karim,
I'd say don't overthink it (Like i did for years and years!). What you're looking for is a trial lesson, call redhill and tell em you want a trial lesson, that will be your first hour of training. After that just tell them you'd like more! Earning a PPL in itself will be a pretty intense course of study alongside the practical training so start soon and fly oftern.
How's it looking in terms of time to study? Reckon you can manage it in the time you'll be in 6th form?