I've made the right decision in terms of numbers- It would take me 45 months (Nearly 4 years) to do the PPL on the volunteering scheme. It would take just under 2 years if funded by a p-t job.
That's a pretty short sighted way of looking at it. In four years you will be 21, and there is no way you would still be legally volunteering on this basis.
Anyone who enquires about a volunteer program, or work experience, should do so with the experience and the future prospects in mind, not the numbers. Otherwise no one would ever do an apprenticeship as the wages are really really low (lower than standard minimum wage) but it's about the experience. You think an apprentice mechanic is changing an engine on his first day? Or do you think he's doing the washing up and making cups of tea whilst observing what is going on.
From the school's point of view, you asked about volunteering, they perhaps didn't have an established volunteer scheme, so simply found you something to do. When you turn 18 and after a year of this volunteering, you could easily have found yourself working behind the bar or restaurant, on a proper wage for your age, and meeting some very experienced pilots and getting exposure to the industry you want to enter.
Rather than sucking it up and pursuing this opportunity, you have chosen to chase a job in retail, which you admit you will hate. This will keep you busy and away from the airfield, and you will find the money in your account is far more tempting to spend on girls and beer, so your PPL will probably still take 4 years anyway.