The registered office test that quietly fails
An address either passes two conditions or it does not. Getting this wrong is one of the few ways a company can be struck off without anything else going wrong.
Three requirements, not one
The registered office has to be a physical address in the UK, and it has to be in the same country the company is registered in. A company registered in Scotland needs a Scottish registered office.
The third requirement is the one that catches people. The address must be appropriate, and appropriate has a definition.
What appropriate means
Two conditions, both of them practical. Post addressed to the company and delivered to that address will be brought to the attention of you or somebody acting for the company. And when post arrives, the sender can get confirmation that it was delivered.
A Royal Mail PO Box no longer passes, and neither do the equivalent services from other providers. If you use an accountant's or a solicitor's address, that address has to meet all of the requirements too, not just the first two.
What happens if it does not pass
GOV.UK puts it plainly: the company could be struck off the register if the registered office does not meet all of these requirements. Not fined. Struck off.
For a company owned from abroad that is a real risk, because the address is usually somebody else's and the company never sees what arrives there.
The address is public and the email is not
The registered office is published on the online register, so a home address used as one becomes a public home address. If that matters, the options are an accountant's or solicitor's address with their permission, or an agent who provides one.
Separately, every company must give Companies House a registered email address and read what arrives at it. That email is not published on the public register, which is the opposite of what most people assume.
The dates for your own company come out of the deadline calculator, and the filing itself is described in the dossier.