Depends on the insurer. Most allow 30 days, but some won't entertain the idea. You'll have to speak to them.
Failing that, you simply need to take out a full policy and then cancel when no longer required. Shouldn't cost more than a £100 or so, depending on insurer and their pro-rata policy. Either way, you'll need to get it taxed or take the risk of getting pinged by ANPR/police. If you price it correctly, it will sell.