We may as well get used to it, it's unlikely to be the last instance of this kind.
To be fair to the police, they were between a rock and a hard place. As soon as they were informed (presumably by ATC or EMA/EXS ops) that there was flight with a declared emergency inbound on 27, doing nothing wasn't an option and closing the motorway was inevitable..
We can't expect the police to be aviation technical experts able to distinguish between various flavours of emergency. Even without the shadow of Shoreham, they weren't to know whether they might be looking at another potential BD92.
Hindsight is a wonderful thing ...