If you receive the message:
"The site cannot be published to the specified location.
Cannot connect to the specified site URL."

or rarely "Internal Server Error"

when trying to publish, the chances are high that the "Web site URL" is set incorrectly, this may have happened if your site has been migrated or if a domain mapping has been swapped.

If you click on the "Edit publish settings" text a menu will appear where you can change the Web site URL. This will either be http://domainname.co.uk or alternatively the temporary URL as given on the main page in the control panel.