RM and the PO have been separate organisations for some time. Also lots of POs are run by independent franchisees as a supplement to their convenience store. I doubt these people want to spend time processing old collections of stamps. I don't know how much they get from RM for every item of mail they process, but it's probably a pittance. RM isn't going to pay them a fee to exchange stamps.
Given the PO scandal which has recently received a lot of publicity, would anyone running a post office want to take the risk of accepting fake old stamps? (Well lots of them already do accept fake stamps for postage, but then the point of this entire exercise is to get rid of fake stamps.)
So perhaps swaps could be done only in those dedicated "crown" post offices - but there aren't that many of these left, and they are always extremely busy. Whereas the RM customer service points are no longer busy, now that people no longer need to spend hours every year travelling and queuing up from 0700-0900 on weekdays only, as postmen no longer collect signatures on Signed For items and just drop them in your postbox (which is a genuine enhancement, and also makes the former name of Recorded Delivery more apt).