A registered office is the statutory address of a limited company or LLP. You must provide registered office details for Companies House when you incorporate a company or LLP in the UK.
Government agencies (such as Companies House and HMRC), will then post all of your official company mail to this address. It will also be disclosed on the public register of companies.
A registered office must be a full physical address (not a PO Box Number) that is situated in the UK jurisdiction where a company or LLP is registered, which will be England and Wales, Scotland or Northern Ireland.