It appears that our concerns are shared by the Police Federation of England and Wales who have, unusually, written an open letter that is linked from here.
They express their anxiety that their members might be open to civil and even criminal liability for wrongful detention in breach of S46 PACE.
Members may want to keep this letter and these concerns handy in the event that any such an ill-thought-out initiative is re-introduced in the future.
This is a link to the open letter on the Police Federation website section-46-pace_open-letter-to-npcc.pdf (polfed.org)