Great News! Nominet (the holder of all .co.uk domains) have updated their system to allow users to purchase .co.uk domains from 1 year up to 10 years, previously it was only possible to register 2 years at a time.
FreshSites recommend purchasing domains as long as possible for SEO reasons. If you purchase a 10 year domain, this is another sign to search engines that your not just a fly-by-night operation and that reputation matters to you.
The changes, which were announced in November, bring .uk into line with generic top-level domains such as .com and .org, as well as a number of other country-code domains.