From the website I can click "Add/renew/top up ticket" then choose an annual travelcard.
You won't want 'annual' though, it's cheaper to use monthly ones (unless you're certain you'll be living in London in the holidays and using it). If you go to a ticket office they can give you any period longer than a month, e.g. 37 days or something, at the monthly rate. This is cheaper than buying a month and a week (for instance).