I have looked into this and so far I can't find any insurance company offering unlimited overseas insurance. Furthermore, if you are a UK home owner, as sure as eggs is eggs your house insurance will be invalidated if you are away from it for over a set period of time.
Nothing to prevent it... yet.
Safeguard used not to impose time or mileage limits, but have had to change underwriters. While I believe that there are still no time limits, at least one of the underwriters appears to impose mileage limits that might be restrictive.