"What kind of security risks are involved?"
The following is pure supposition (on my part)
I have posted before that there are a number of areas in the northern wildernesses that are off limits to the public, albeit unofficially. I know of 2 in the Dales, 1 on the moors and 2 in the (wider) Lakes ..... there may be more. These are valleys with a restricted entry which can be sealed off by the simple action of closing the entry road. The reason is that the houses in these areas have been commandeered for retiring "bigwigs" who must avoid the limelight.
In the recent past there has been a wedding in one of these valleys. The father was a high ranking officer and there were several other "targets" including senior politicians there. The access was closed for "drainage work" which prevented public usage.
There are also a number of usable Auxiliary Unit bunkers remaining, one accessed from within a greenhouse, another below a grouse butt. Two more lie close to the Coast to Coast walk.
Again, some Starfish sites remain which are occupiable ... though whether ever used by service personnel I don't know. One I can mention (as it is no longer used) is at Percy Cross south west Guisborough, less than 1 mile south of the Cleveland Way if anyone wants shelter for a night!
I BELIEVE, though can't confirm, that these are available for specialist usage. Should special forces therefore use them the danger lies in their discovery rather than danger to the personnel. I haven't located them yet though I have been told where the greenhouse is.
How do I know? These are rumours that you pick up from local farmers or historians in the area.
I have no military experience so apologies for technical inaccuracies!