The written exams and pass standard are substantially the same for the two, and few people will reach that pass standard in less than the PPL minimum 45 hours. So, in your position, I'd recommend the PPL so long as you can get a class 2 medical.
Pretty much all schools permit you to pay as you go, and most people on here will recommend that you take that option. Money paid up front has a bad habit of vanishing in this volatile business environment.
NW London - try West London Aero club at White Waltham, Wycombe Air Centre at Booker (M40 J4), or The Pilot Centre at Denham - all good well regarded schools.
G