All .uk domains, including .co.uk, currently renew for a fixed 2 year period. However, from May this year it will be possible to renew for 1 to 10 year periods. It will also be possible to renew domains before they expire (unlike currently).
– It will be possible to secure domain names for longer.
– It will be possible to get discounts by buying longer renewal periods.
– It will be possible to show to Google that you’re in it for the long run by buying 10 year renewals. This only affects new domains – probably.
Remember, you will be able to renew in advance even if your domain is not due for renewal now.
From an SEO point of view we’d recommend you renew for as long as possible… just don’t forget to renew in 10 years time!