Can I use a PO Box as a Registered Office
The answer is no.
All UK registered companies are legally required to have a UK registered office address. It is the address of a company to which Companies House letters and reminders will be sent. HMRC, the UK tax authority, will also contact the company through this address. The registered office address can be anywhere in England and Wales (or Scotland if your company is registered there). It should be a genuine location and suitable for delivering documents to the company. It is also the address at which legal notices are officially served.
The address must be effective. This simply means the address must be accessible and people should be able to deliver formal notices to the company at the address. PO Boxes are not suitable for this as they do not provide a detailed address with building number and street name.