Yup Teams works for me.Are you a sole trader, or a limited company?
A Royal Mail PO Box would cost you £424 per year. But if you're a limited company, Companies House will no longer accept a PO Box as your registered address (since March this year). Without that, I don't believe that D&B will show your PO Box address, and without that I don't think Apple will accept it. So, just use your home address (I do, it's fine, why exactly do people not want to do that?). If you're a sole trader it will be different; I can't comment on that.
A telephone number... so far, I don't think anyone has reported successfully using any kind of VOiP or virtual phone system with Apple's number verification. I tried with the AWS thing and it didn't work. (Did any of the people mentioning Microsoft Teams actually have success using that with Apple's verification, or was it just a suggestion?). So I think the best option would be to go to Tesco and buy their cheapest SIM card; maybe £10? That's what I did (see previous posts). If you actually plan to answer the phone if someone calls (I don't!), you also need a phone to put the SIM in - if you don't have a spare one, back to Tescos!
Answer to your question: £10, or £434, but more importantly it wastes a lot of time!
In the UK you can get a legal VOIP number (all telephony is now voip) for like a whopping £1.20 per month