Hello mate, sorry to hear about your back!
I've used Bupa a few times (3 operations and 3 lots of physio). Gotta say, its really good, much better than waiting for the NHS. Only problem is they increased the excess a few years ago, the last time I used it, it was ÂŁ150 IIRC.
The way I had to do it is to see your GP, and tell him you are with Bupa. He will then make you an appointment with a local hospital. Get on to your firm and ask for the JIB/Bupa phone number (if they dont know, try the JIB themselves). Phone them up, and ask them for a claim form, you might want to check with them that what you are having done is covered, also make sure the hospital you will be going to is part of their 'scheme'. I think you have to take some paper work with you when you go for your appointment, so the hospital knows who to charge.
Just be aware that the JIB/Bupa scheme seems to be slightly seperate from Bupa itself (I presume there are lots of different schemes or partnerships). Just make sure that you are covered BEFORE you get anything done, otherwise you could be in for a nasty invoice
You will get several invoice's after you have had your consultation, operation, and any physio or follow ups, but these will all be paid by Bupa (its interesting to see how much doctors charge!). From memory, I think I had to pay for some of the anethnetis(sp), as he wasnt covered by bupa, but it wasnt too much.