The Royal Mail need a kick up the backside. I complained via their 'contact us' website form back at the beginning of April. Today, I received a 'response' that basically just said "please call 0845....". They didn't answer my questions, nor did they even try to resolve the original problem.
I called 0845whatever, and got told there's no way to have a properly 'location independent postal address' (like a national PO box or whatever). Thus a PO Box is only useful to 'hide' your actual address from your contacts - NOT to help you maintain an address even if you move.
Just as you've probably come to expect of me, I replied to the 'response' complaining about it, and again asking how I can resolve this issue (and if Royal Mail ever intend to offer a resolution). Of course, I don't hold any hope at all, but there's no point moaning unless you do something about it.