It all depends on your GP, and, as has been said, your PCT.
You need to be at a healthy BMI, or almost there, before you can be put on the waiting list.
Basically, your GP's your first port of call, and he/she will decide whether or not to refer you to a surgeon. Your surgeon will then decide whether or not your a good candidate (healthy weight, healthy in general, have enough loose skin, etc). He'll then write back to your GP, who will submit a request for the funding with your PCT. If they say 'yes', then you get put on the waiting list.
It's all a bit of a lottery, depending on where you live and what your GP's views are. You won't actually need to 'prove' that you've lost a lot of weight, as your tummy will be a reflection of that if you have enough loose skin. You don't need to prove how long you've been maintaining either, so could technically make that up I guess, so long as your GP hasn't seen you for a while so doesn't know if you've been recently overweight!
Hope that helps.
Jo x