A couple of points - first of all, you CAN just take out buildings insurance, but be aware that this may not cover some of the fixtures in your house eg carpets, curtains etc

Second you can take out a monthly policy, many companies do these and you pay monthly, the cover is in force as long as there's premium paid that month, so when it's sold you can stop the insurance - that would solve that problem - no need to have an annual plan. (But the premium is higher than normal policy)

A reputable insurance agent would be able to help you find the right company if you cannot get low cost monthly cover from a major insurer.