I really wanted to know if this was true so I've searched and found this from the Post Office Act 1953:
Criminal diversion of letters from addressee
(1)If any person not engaged in the business of the Post Office]
wilfully and maliciously, with intent to injure any other person,
either opens or causes to be opened any postal packet which ought to
have been delivered to that other person, or does any act or thing
whereby the due delivery of the packet to that other person is
prevented or impeded, he shall be liable
on summary conviction to a fine not exceeding level 4 on the standard
scale or to imprisonment for a term not exceeding six months or to both]
in this section shall apply to a person who does any act to which this
section applies where he is parent, or in the position of parent or
guardian, of the person to whom the postal packet is addressed.
So it's only illegal if you're doing it "with intent to injure" i.e to steal someone else's mail, not if you're trying to help them, which in effect you are. I don't know if that's necessarily the correct interpretation but that's how it seems to me!