Yes, you can use a PO Box as a trading address. A lot of small and home-based businesses do this because it is cost-effective and protects the privacy of the business owner’s residential address. However, some HMRC departments do not accept PO boxes so you will have to check with them.
A better option is to use a business address service. You can get these from lots of places, including company formation agents and secretarial services firms. This is often a better option because your mail will be forwarded to you as part of the service, so you won’t have to personally collect your mail.