1) UAS is not the way to PPL if you are not interested in military flying - you are taking the place away from someone else.
2) All Single-Engined Piston hours are counted towards the PPL, but the requirement still have to be fulfilled - this includes more medium-level nav, VHF RT (not much different), square circuits etc, most ground exams that are not covered by the UAS.
3) Unless you are joining up, you won't be in the UAS for 3 years - 2 years in the maximum before application for the RAF is encouraged. With good weather the syllabus could be completed - with poor you could only have 40 hours at the end of it.
4) There was an agreement with the CAA I believe to award the PPL with a couple of extra sorties and tests, though I don't know what became of that - perhaps ask in Military Aircrew forum.
PS it's not 'free' flying any more - you have to work for it.