Elstree wouldn't authorize me to depart as they hadn't received SB approval
Is this a police state we live in?
Firstly, Elstree have no business getting "approval" from anybody.
Secondly, the cops have no business issuing an "approval" because there is no law which supports this. The word is NOTIFICATION.
I know that the coppers like to phone you in the middle of the night with a "permission number" but that is a job creation scheme which has no support in the legislation.
A 12hr notification is what is required, and then you can go.
They can arrest you of course; a copper can arrest anybody without an explanation, but short of that they can't stop you flying if you have given the 12hr notification and can prove you did it (e.g. with a fax receipt).