We do NOT recommend UK2.net as a domain registrar for SupaDupa because their software interface does not allow the configuration of a domain name in way recommended as compatible for SupaDupa.
Specifically, the UK2.net software interface does not allow the configuration of a non-www redirect to the www domain, meaning that visitors who type in your domain name without the www will not land up on your SupaDupa site, which could end up losing you sales.
Please also note that to get even the most basic DNS editing ability within UK2 they will charge you extra for that, whereas other domain name providers will offer that as part of their domain registration cost.
If you still wish to use UK2.net knowing the above, then to configure your domain so that www.YOURDOMAIN.com works, you need to:
1) Log in to your UK2.net account here: https://controlpanel.uk2.net/index
2) After logging in, click on "Domains"
3) On the list of domains page, click on your domain name.
4) Click on the "Change contents of Zone" link. Please note if you do not see this link, you will need to pay an additional fee to UK2.net to activate the feature.
5) On the edit DNS zone page, look down the list of Custom Records until you find the row with "www" in the left hand box. The drop-down select on that row should say CNAME. In the right hand Host/Data box, type in "mysupadupa.com." (with trailing period).
Please note that it is exactly "mysupadupa.com.", NOT THE NAME OF YOUR SUPADUPA Store.
6) Click "Update".
7) You can now go to your SupaDupa admin, click "Shop Settings" and choose "Domains", and create a new domain for "www.YOURDOMAIN.com". (it must have the www in front).