Steve,
There are plenty of medical/life insurances available here in the Philippines. The costs per year really depend on your age, existing medical conditions and what type of coverage you want.
A plan that pays in the 3-5 million range costs around 50,000php per year.
One of the plans we have is Caritas health insurance. Might be worth looking at, as I know they insure people that are over 60. Not that you are....yet!
I agree with what Michael says about Philhealth. Without looking I think we pay around 200php a month. They usually pay out around 20% of the bill, however I have heard it can sometimes be less or more.
DO NOT presume you can just fly back to the UK and get treatment if something is seriously wrong. A medical condition may make it impossible for you to fly.
Most expats I know tend to pay for their medical needs out of their pocket, rather than having insurance. T