Hello Jon,
Welcome to eUKhost forum
You can simply point nameservers of your domain name at 123-reg to the nameservers we have sent you in the welcome email. You can add a separate account for your domain with 123-reg in your WHM with us. You can transfer your data to our server and your domain name will work from our servers.
I don't think you should opt for forwarders as that doesn't look good in front of visitors. You can get the domain name transferred from 123-reg to us as recently 123-reg service has given lots of problems for other customers who have come to us and one of our customer could not change his nameservers for close to 1 month. If you manage to change the nameservers easily from your 123-reg control panel then you wont need to transfer the domain name.