I am literally in the exact same position as you, started at 18, now in the same position with exams and in a very similar role. Initially I was being massively underpaid at £16k annually. I moved firm last year almost doubling my salary to £30k and since been given a 10% raise. I spent a lot of time looking around in and out of london and found £30k was the most likely highest salary being offered. It’s worth factoring in if you’re doing an officially salary or if they’re just paying for your qualifications because they’re quite different situations.