Andy, I word from the wise.
My mum had a gastric bypass 4 years ago and lost 17 stone. She's only 4' 11 so you can imagine the picture.
Her GP referred her to Plastics, the waiting list was HUGE and when he appointment came around (I think she waited 4 months) she was told that because of the cuts, our PCT could do the surgery. Apparently they wouldn't do it because it was cosmetic.
My mum and the GP appealed, it had to go to panel and anyway she got it and goes in for surgery on Thursday this week!
My advice is to push with your GP and the Plastics when you see them that your desire to have surgery is not purely cosmetic. You need to stress that the excess skin either inhibits your day to day functioning and / or is having a detrimental effect on your menal health (depression, low self esteem, not socialising? etc)
That apprrently is the criteria now, it has to have a 'functional' or 'psychological' on your quality of life.
We wish we'd of known this at the start, it would have saved us 18 months of hassle.
Bear in mind though an apronectomy +/- other bits is MAJOR surgery. Just be sure it's what you want.
Good Luck.