Your domain doesn't have to be registered with us in order to use our hosting service. You will however need to point the domain to our servers for your new hosting package to work.
It's quite simple to do:
1.) Log in to your current domain provider account
2.) Look for "DNS Settings" or "Manage DNS" (something along those lines!)
3.) Set up Custom Nameservers to ns1.cloudhosting.co.uk & ns2.cloudhosting.co.uk
It can take up to 24 hours for your DNS settings to update, but in our experience, it's usually done much faster than that (sometimes in a few minutes!)