[Replying in general]

The clause in the General Terms states:

Where a parcel post article, an express post container or an express post parcel is returned to its sender pursuant to this clause and its return entails its carriage from the office at which it was held for delivery to another office, postage is payable on that carriage as if it were an original carriage between the office at which it was held for delivery and the return address of the sender.

So, I think helen is right (in that it's the PO or LPO that applies the charge), and gather that if a parcel is successfully delivered then marked RTS and dropped off in a red street box, or refused at a physical address, rather than held for collection and returned by the holding office, the charge won't (or is unlikely to) be applied. I've had a few parcels that were deemed undeliverable (insufficient address) and immediately RTS, refused delivery, or accepted then marked RTS, and none of those came back with a charge - the parcel referred to above was awaiting collection at a PO for a while before being RTS.