You need to contact the Post Office to get a P O Box set up, don't know if they would ley you choose the number though, the one you want may already be in use!
Your PO Box� address
You�ll be given the next number in sequence for your local sorting office. We may have to change PO Box� numbers from time to time, but we�ll always give you reasonable notice.
So it doesn't look like you can choose the number.
= "we may have to change your number from time to time " means: "if someone is willing to pay us extra for your number we will take it and dump you with another one"