The official line may be "18 weeks" but there are ways of surgeons getting around that deadline!
My PCT (Calderdale and Huddersfield) make an initial referral to the surgeon (after funding has been granted) who will have a consultation with you, order a whole bunch of tests, and later have a meeting with other surgeons to decide what is the best course of action for you. If you’re given the go-ahead for the surgery you are referred back to your original GP, who will re-refer you back to the surgeon. Once you have had the second consultation with the surgeon the 18 week 'deadline' applies.
I strongly believe all this referring and re-referring is simply to cut the 'official waiting times' and to ensure they are adhering to government set targets.
When I mentioned this process to the GP who referred me she got really angry because it costs the NHS every time any referral is made!
This being said, however, each PCT has its own processes it follows to ensure this target is being met. You should really contact the PALS (Patient Advice and Liaison Service) for you PCT to get a more accurate picture on the realistic waiting times.