They need it for tax purposes.
but I don't think the networks ask me for my tax id because I'm from america, I'm not from the UK.
That's what I mean. If you live in the US and are using US networks, they are going to want an address and your tax id or social security number.
I'm not sure if you can use PO boxes for any aff networks but you're going to have to give them some kind of address to mail any pertinent tax forms at the end of the year.