PPL part time?
I did my PPL part time from May 2012 to June 2013. I was flying near London (Stapleford) and lost a good chunk of time to Olympic flight restrictions and weather. The weather was the most frustrating as availability was quite tight both for my instructor and for me working every other Saturday and improbably lost about a 1/4 of booked lessons.
I think without working weekends and Olympic restrictions and a small amount of luck with wx, you should've able to finish by early autumn if you fly every weekend.
I'd second the advice on getting the exams started ASAP - you probably can't solo until you have air law - and also get your medical done ASAP (get class 1 if you plan to go commercial, class 2 if not)
I did make the decision to pay up front - many people for good reasons suggest not doing this but I paid on credit card and think I would have been protected if anything happened. Also the school is well established with lots of aircraft and commercial students and a good track record. Good luck and enjoy. I did.