Often, for a reduced cost, health insurance companies (such as Health-on-Line) provide a '6-week option', which implies that you go through the NHS if treatment is available http://www.saga.co.uk/insurance/health-insurance/personalise-your-health-insurance.aspx within 6 weeks, and through private health care if it's not. This permits personal medical insurance to serve as a safety internet, not a replacement, for the NHS.
Furthermore, some advanced treatments just aren't available on the NHS, because they're considered too expensive to be reliable. This implies that specific cancer drugs, such as Herceptin and Avastin, can not be recommended private medical insurance, and neither can experimental drugs and treatment not yet authorized by the National Institute for Health and Care Excellence (NICE). These forms of treatment are only available through private health care.
Vitality have a special offering in the UK Health Insurance market. Their company offers a Medical insurance policy like the others however they integrate it with their Vigor system - a set of discount rates and rewards for healthy activities or items. Vigor's thinking is that if you are healthier, then you'll assert less and in turn they can then charge you less too.