A quick Google will show there are a few companies that provide health insurance. The cover you decide will depend on so many things… but we are looking at True Traveller and World Nomads. Both of these companies have had some good reviews on various Overlanding Forums.
We decided to take up travel insurance with TrueTraveller. We chose TrueTraveller as they will renew while out of the UK. Worth noting that you cannot start or renew the policy once you are 66 years of age. There are other companies that will provide cover beyond this though. This was for worldwide cover.