Hours count towards PPL as you ARE being taught by service QFI. Flying you may do with AEF pilots doesn't count (i.e. Non instructional flying). There may even be the opportunity to do you qualifying x-country as well.
What do you mean by you can't commit yourself full time? To the UAS or an RAF career? With the UAS there is a commitment of one evening per week for ground training AKA Town Night (You go to your Town HQ and have a chat from someone in the services and then have a drink.... or 10). You also have to do 2 weeks of camp, normally during the summer. That is the minimum requirement. But if you just did that you might find yourself getting chopped as many studes attend during the week to fly and go on all the sporting and AT expeds which happen at weekends and during the hols.
You may find that your UAS also require that to stay on into the 2nd or 3rd year that you have to submit your RAF application for either DE or a bursary. Does vary though. Hope that helps.