Been with BUPA for 12 years now on the RAF Discount scheme and have had excellent value for money. Never any debate and, although they might not authorise full treatment at your first request (IOW. Tests and consultation first; then scans and X-Rays; and, yes once surgery required). Definitely had my money's worth for a family of 6. Of course, like any insurance policy, their monthly premiums increase with inflation. More importantly perhaps, like car insurance claims - the more you claim the more your premium increases. Also, the older you get, unlike a car, the more you pay in premiums as you are more likely to fail!
A few words on the Private Sector! It's not all it's cracked up to be. Yes you jump the NHS queue, but the consultants are the same guys (and gals) that you would see on the NHS - you are just paying for their golf club fees, their mansion in the posh part of town and the Bentley on their drive. Also, although you might get a nice welcome at a Private Hospital, a room of your own, wine with your dinner, priority treatment and a smiling and apparently happy nurse, the behind the scenes is not always reflective of that environment (I know, I spend most evenings hearing about the goings on from my wife who nurses on the Private sector). The first priority of a Private Hospital is, like Private Nursing Homes, to make money! Patients come a very low second and the staff are poorly paid dogs bodies in pretty frocks (that's why Mrs FW agencies at double the payrate!!).
On balance, I would go private everytime. I have and so have all my family been treated quickly and efficiently despite the behind the scenes described above.
Last point, under BUPA, no child over 21 can remain on the family policy!