I was in a similar position a few years ago. Any time spent in the military gives good, sought-after transferable skills, so I think you are wrong to dismiss those that initially do not offer this. Anyway, you are looking at a minimum of 12 years (RAF Aircrew short service commission) until this comes in to play anyway, and that is plenty of time to get skills that will be useable in civvy street. Plus WSO/Obs will give you as good if not a greater chance of civvy employment than any of the other trades you mention such as Army REME mech.
As for what your parents think, they will get over it and be proud of you whatever you do, you cannot let that hold you back from what you want to do.
Decide what you want out of your time in, and then go for what you want to do. You cannot base it on what will please the 'rents or what will get you employed in civvy street after you have done your time.