Generally Long Stay/Gap Year type policies will cover you for one trip only with cover expiring immediately upon your return to your home country regardless of when the insurance was due to expire. Some policies may allow for a temporary return to the UK but it is unlikely that this would include using the policy as a multi-trip insurance for the remaining period of cover.

Despite this if you believe that you will be away for up to 12 months then it would be worth taking a policy for this period as you may experience difficulties extending the cover if you only take insurance for 6 months and then decided to stay for the full 12 months.

Hope this helps.