By CA 2006, sec86, a company must at all times have a registered office to which communications and notices may be addressed.
When a new company is registered the address is specified on the initial registration forms. Any change in the address must be notified to Companies House on Form AD01. (Forms can be obtained from the Companies House website.) The change takes effect on the date the notice is registered, but any document may be served on the company at the old address until 14 days after that date (CA 2006 sec86(2)).
The registered office must be in the country in which the company is registered: a Scottish company may not have its registered office in England or vice versa. There is, of course, nothing to stop a Scottish company having a place of business in England (or an English company having one in Scotland), but it must maintain a registered office within the country of its situation.
The address of the registered office must appear on all business letters, emails, websites, etc.
Incorporation Services Limited provides a registered office service.