For me, the two seem to work hand in hand. BUPA through work by the way.
Knee problem, see GP, ask for private referral to local BUPA hospital, get approval from BUPA (two consultant visits plus MRI approved straight away), appointment for < 1 week later, MRI next day.
There is an excess to pay, which in our case is 200 pounds.
EDIT: Had my other knee serviced 10 years ago. The overall bill was about 8000 pounds, of which I paid 100 pounds as the excess at the time.