On 123 Reg you will need to go to the domain control panel, i assume they have one, and enter the DNS details provided in the signup e-mail that euk sent you.
This will be something like:
ns15.eukdns.com
ns16.eukdns.com - Just and example
Depending on registra, this can take 24 - 48 hours for these changes to take effect and your
website hosting to show.