From my experience once you have been refered by your GP the three stages are
1. Group meeting to give you the information you need to need to make an informed decision.
2. You then will have an assessment with a specialist nurse to see if you are right for the procedure. they will want to check you are commited to the lifestyle changes that you have to make. Remember this is a huge investment on behalf of the NHS and they want to make sure you will do your bit.
I believe some trusts will only intially provide funding for assessment. You then have to be "Passed" by the Bariatric nurse before you get funding for the operation.
3. You will then undergo medical assessment to check you are physically fit enough to survive the operation.
Once these steps have been taken you should get a date for the procedure