Company Registered Office - Can I Use A PO Box?

Short answer: no.

A UK Company Registered Office has to be a bricks-and-mortar address; a building which has a physical presence. If this were not the case, anyone could set up a company with an intangible presence; it would be 'located' in a mailbox, somewhere. Which isn't really existing, at all.

This would lead to an increase in criminal behaviour. People would offer free-celebrity-diamond-iPad-loans for one low downpayment, then skidaddle when the authorities started taking an interest. There would be no place to go to get answers from the owners.
 

PO Boxes are great as a place to put on a website for a competition. You have an address which is separate from your main business where Joe Punter can send in his coupon for a change to win a Magnificent Prize. This means your main address only gets the mail you need to run your business.

They are great also for a person of no fixed abode, to put as their mailing address; one travels the world for six months of the year and collects one's mail when, and if, one wants.

A PO box is a 'post office box'. Someone who doesn't have a normal home e.g. a backwoodsman,  can collect their mail by riding into town on their horse once a month, mosey on down the the post office, and pick up their correspondence, along with some supplies.

It can be a literal mailbox, with a key. More commonly, you just get a number like PO Box 487 and a nice lady named Bertha hands you your mail when you show your ID. Royal Mail offer them as standard.

A company, on the other hand, has to be accountable;  those who deal with it have to be able to send legal documents to it by mail, and to present them in person, if necessary.  PO Box numbers inspire no confidence in anyone doing business with you.

The UK company registration system is less bureaucratic than other nations; it's fairly simply to set up and run a UK company. You just need one man and a UK address where you have the permission of the landlord to receive statutory letters.

It stops short of being purely virtual, however.

The address has to be in the UK, and to be 'real'. You can't just set up a website, call yourself Towering Ego Ltd and expect to enjoy the legal benefits of being a limited company. Consumers have different expectations of a limited company than they do a sole trader.

→ See how much a London registered office would cost you