Chiming in as I have read all the relevant legislation surrounding buying, selling, and posting kitchen knives in the UK (and I work with policy and lawyers for a living, so I feel remotely qualified to comment).
Basically there should be no restriction on an average person buying, selling, or posting a kitchen knife as a private sale provided it is packaged safely enough for it to be processed during shipping using a service that requires an 18+ signature on delivery.
DHL have long been known to turn kitchen knife packages back to the sender when importing into the UK, and I can only guess this is because their legal team are overly risk-averse and have adopted a blanket policy which goes over and above any current legislation.
It now looks as though Royal Mail are adopting the same approach.
All this means for now is that you won’t be able to ship to the UK using your national post carrier. All other couriers (but not DHL) will be fine provided you follow the requirements about packaging and 18+ signature.
Parcelforce is a separate entity to RM and haven’t announced any changes to their policy as far as I’m aware.
I would be surprised if other shipping companies in the UK adopt this approach, and many here just see it as another small step for RM fading into obsolescence.