I've the next entry within the httpd.conf

<VirtualHost *:80>
    ServerName www.mydomain.com
    RewriteEngine on
    RewriteRule ^/(.*) http://www.anotherdomain.uk.com/ [L,R=302]
</VirtualHost>

I'd turn to arrange this so that when entering www.mydomain.com/admin I won't get rerouted.

<VirtualHost *:80>
     ServerName www.mydomain.com

     RewriteEngine on
     RewriteCond %{REQUEST_URI}  !^/admin
     RewriteRule ^/(.*) http://www.anotherdomain.uk.com/ [L,R=302]
</VirtualHost>