Prices vary. Fasthosts currently charges £9.99 + VAT per year for .co.uk domains, £12.99 + VAT for .com domains. They often have a special offer and sell the first year for £0.01. (This price was checked on the 8th of June 2019)
.co.uk Domains are registered for a mininum of 2 years and renewed for at least another 2 years. However, you can buy credits at a discount and register your domain name for example for the next 10 years. Saves you money and you don't have to think about it for a while.