This can be a problem area. I think most will only cover up to 3 months.
There is a company called hertsinsurance.com that provide a walkabout type Insurance policy for those that require longer period of cover while abroad.
The following is from the DVLA site.
Taking your vehicle abroad for less than 12 months (temporary export)
If a UK registered vehicle is taken abroad temporarily, it remains subject to UK law. This means that you as the keeper, must by law make sure that the vehicle stays taxed while it’s overseas. Providing the vehicle has a current MOT certificate and insurance, you’ll be able to tax the vehicle.
If you don’t tax the vehicle and it’s brought back to the UK untaxed, the vehicle will need to be transported and not driven upon entry back to the UK and SORN (Statutory Off Road Notification) should be declared straight away.
You can tax or SORN online or by telephone, but please note that SORN can’t be made while the vehicle is abroad.
If you don’t have a registration certificate and you are taking the vehicle out the country on a temporary basis you can get a replacement from DVLA by phoning or applying by post.
Hope this helps.