But it would probably have saved her life though. Even surely wouldn't have been sufficiently brazen/stupid as to have carried through with his kidnap/rape/murder/burn scenario if he knew he'd been positively identified before he was even able to put cuffs on her and place her in his car.
Also, bear in mind that the guidance applies to one person on his/her own, in plain clothes, who identifies as a police officer. Obviously, the chances of two officers acting in cahoots with each other to abuse their power in such a grotesque manner are way, way lower than with a single officer acting alone (and of course the chances of even a single officer acting in this way are already extremely, extremely low).
Plus in any case, I believe that for a long time it's been standard procedure - in London at least - for anyone stopped by a police officer in this sort of scenario to have the right to walk with them (if on foot) or drive (if in a car - with the officer following in his/her vehicle) to the nearest police station.
I believe that Sarah Everard could, had she been sufficiently aware and astute*, have made such a request of , and - had he been acting in accordance with proper procedures - he'd have had to comply with that request. Had he indicated non-compliance (as indeed he might have done in this instance), then Everard would have had the right to refuse arrest and to either phone for assistance or flag down a passing car, etc.
* But of course one of the (many) chilling factors of this awful crime is that Couzens would have known the immediate advantage he had over someone like Sarah Everard: that he was the authority figure in their interaction, and that - like many (even most) people - she would have found it very hard not to defer to his authority. On top of that, she'd have been experiencing fear and probably an element of panic at hearing she was being arrested, with all that that entails, and again this can push someone into a state of compliance. If there's one good thing that may have come out of this sick tragedy, it's that in future hopefully it will be very much more difficult for this sort of thing to happen again.