What a lot of questions, many of which I can't answer.
However, be aware that the 15 hours flying between ADC1 and 2 is for NATS-sponsored courses only.
My IMPRESSION (and it is no more than that) is that NATS is not terribly keen on taking on people from outside when they have a surplus of trainees trying to get into the airports. I can't speak for the non-NATS units.
Forget any ideas about trying to get into Area Radar self-sponsored. Other than a transfer from the military or occasionally from other countries, it won't happen.
Some NATS aerodromes are going down the road of tower-only now, with the approach units located with the radar unit at West Drayton (this includes Heathrow, Gatwick, Luton and Stansted and maybe Southampton one day?). Beyond that, many units start with tower validations and then extend you onto approach as need arises, but the cost of an approach/approach radar course makes this a big undertaking.
Finally, I don't know how often/whether the College of Knowledge runs courses to which self-sonsored people can apply. They seem to contract out to other countries such as Netherlands, and I don't know whether you would be able to tack onto one of them.
Not much help, I'm afraid, but good luck with whatever you decide, but be prepared for a long haul.