If you require a domain name, that is www.something.co.uk, it is included in the price (if it is available, of course). Sometimes common names have been taken, so consider alternatives. For example if you want www.paddysbar.co.uk, it will almost certainly have been sold already, so you could change it to www.paddys-bar-leeds.co.uk, or something similar.
If you already own a domain name and you want to use that, no problem, you just get it "pointed" to our nameservers (we will give you the details) and we take care of the rest.