A registered office must be a physical address that a person can enter and deliver mail to, so you cannot simply use a PO Box number. You can, however, use a PO Box if you can provide the full postal address (incl. building number and postcode) of the building in which your PO Box is situated. It sounds a bit odd but these are the rules!
You may wish to consider using a registered office service from a company formation agent or address provider such as Mailboxes Etc.