I can see the sub-server page when I look at it from Preview Website. But when I directly look by sub-server URL, it says Virtual host not found on this server
Hmm, “Virtual host not found on this server” is an unusual message. That’s not something Apache generates.
Are you certain that “mydomain.co.uk” has the correct nameservers, and that they’re pointing to your server? You may also want to verify that when you do a DNS lookup on subserver.mydomain.co.uk, that the IP addresses returned point to your server.