I cannot see a way to associate an external Ip address with the domain basename - i.e. "domainname.co.uk". Increasingly these days URLs do not include the "www",
you should also note that browser tend to hide the www. and http or https these days, so this could be a wrong impression too
however, i do not understand what you where trying, redirecting to external server, proxypass to another lan server, or simply change the default domain of the server to avoid the hostname ?
for the last it is simple :
config set DomainName yourdomain.tld
then signal-event domain-modify and email-update. be careful it might also need some modifications for the SSL certificates