Those HSL warning signs are - I am pretty certain - for the UPVCE test site which is somewhere up that way
UPVCE you ask ?
Stands for Unconfined Percussive Vapour Cloud Explosion.
Set up when UK went over to natural gas and the HSL and the gas people had to define safety limits for a cloud of gas being released and hitting an ignition source .
This set the spacing between process units in a distribution centre .
Through my insurance work I saw a HSL film showing these tests - BLOODY SPECTACULAR !!!
Ka-bloody-boom !
Now used by the petrochem industry