S.A.S.
I just don't believe any of this is for real. Much better brains than me have had a look at the legislation and nothing is required within the UK, and also no permission is required.
The Plod have the powers to stop you going, sure, but they will (eventually) have to come up with a reason for having in effect arrested you.
The last person to ask for an interpretation of the law is PC Plod. Like army recruits, they are not picked for being PhD material. Those half smart go for CID.