Too much paranoia
Based on literally over a hundred years of UK history "two reporters from the Daily Mail" present a much greater threat to British way of life than a random group of wannabe Islamic terrorists.
You really can overdo this kind of paranoia. (Anyway, the fiber's primarily for high speed stock trading; some might say that disabling it would be doing society a service.)
Incidentally, the first "interception of cable traffic" type spy story that I know of was a tale by William de Querx, a prolific writer of Invasion Literature. What we have here are a couple of the Kaiser's spies who have branched a North Sea telegraph cable just off the Norfolk coast -- lots of Boy's Own type stuff, a good read if you like that sort of thing. There's even a Daily Mail angle in some of his stories, like "The Invasion of 1910" which used as a backdrop East Anglian towns with a significant Mail readership. (His vision of WW1 turned out to be nothing like the real thing.)
Another literary quirk -- before he and his ilk discovered the evil Kaiser the Bad Boy Of Europe was Tsar Nicholas II. Plus ca change.....