you can't.

you can on occasion but you will never be lucky enough to get a promo and best chance is to get it on a sale when its like $1 only few ppl do this depends

You can get a free one for a year here: You must login or register to view this content. Unfortunately, you have to use their site templates and hosts (Kinda pointless, but the only way I know of getting a free .co.uk domain).

EDIT: After 60 days, end-users will be permited to transfer their one-year .co.uk domain name away from Yola, subject to UK registry requirements.