UK home insurance companies, when quoting, will ask you how often your home is left unoccupied or unattended. These type of policies are only really designed to cover you if you leave your home empty for less than 30 days.
If you have a building that is likely to be empty for longer, you need to think about a specialist unoccupied building insurance policy. You basically have two types. Firstly, those that are slightly cheaper, but offer very restricted cover ie fire, lightning, aircraft, explosion and property owners liability only.
Secondly, if you look around a bit more, you can find a more expensive policy that will offer slightly wider cover, including theft, malicious damage and limited storm.
We would always recommend that you speak to a business insurance broker when getting a quote. Commercial insurance companies offer these, long term unoccupied, policies more than a standard household insurer.
The cost difference is only around 20/30%. In our opinion, given the likelihood of storms in the UK, it is much better paying a few pounds extra for the increased peace of mind.