The London market for accountants isn't as strong as it used to be, don't get me wrong its still good, but there is a lot more competition even for starter roles now than when I joined the profession 7 years back. Like you I made the switch from Retail, and while there are bad days in the office, it is a far more rewarding and challenging job than retail and there is a chance if you're good you can really go somewhere.
I know when we were recruiting for a starter role within my old firm 2 years back we received around 15-20 CV's and we only went to a single agency. There were a lot of good applicants with degrees etc, and I imagine a lot of places would use that as an initial filter. However we ended up lumping for a guy without a degree, simply because he impressed us most at the interview. I think you're going to need something to make you stand out, doing your exams while working in retail is commendable but you may find it hard to make the jump. I only managed to make the transition because I got lucky.
You'll only get £40k part qualified in London if you're either very good, or very lucky, and you'll need at least a couple of years of on the job experience for that. Fully qualified should see you in the £45k+ area quite comfortably, as long as you've got some good commercial experience behind you, and a bit of sense.
Exams are important, firms use them as an easy way to grade where you are in your career, but they are not the be all and end all. We had a girl at my last firm, fully qualified ACA, Maths Phd and she was completely and utterly inept. She is still on the salary she was on when she started, under £30k, and to be perfectly honest that is far too much. Also be aware that to qualify you need to have 3 years professional experience under your belt, so if you keep flying through the exams you may find yourself with a long wait at the end before you can confirm your qualified status.
Can't think of anything else to add, feel free to hit me up with any questions.