You need to do decently at A-Level/equivalent (most of the good unis want AAA-AAB), apply for a BPS accredited psychology degree (you can look at the league tables for rankings) and achieve a first or 2:1, gain a few years of paid and/or voluntary relevant work experience (in mental health + psych research), and you finally apply for the highly competitive clinical psychology doctorate that takes 3 years to complete.
The earliest you could possibly become a qualified clinical psychologist would be 7 years from starting your undergraduate degree (3 year bsc + 1 year experience + 3 year doctorate) and that's a bit unrealistic because most people applying for the doctorate have much more than one year of work experience.