OFCOM allows you to keep numbers only for the purpose of transferring to a new provider, not for transferring between accounts with the same provider. As such, you never own a number, you lease it as part of your package & when you give that package up the number has to be released unless you go to another provider.
A member of staff will confirm, but usually the only way to keep a number like this is to change the name on your father's account to yours.
PS. I have moved your query to the Home Phone support board, as Forum Staff do not monitor the Chatter boards.