Re: Cut off the UK?!?!
Kinda. Look at the Telegeography maps for a rough idea. Some fibre is routed via the Chunnel, but there are.. wayleave issues and other complications with doing that, mostly commercial. But there are fairly widely diverse routes across the pond, and from the UK into Europe.
Challenge from a security standpoint though is I think too many people now know where cables are to too high an accuracy. Anyone who's worked on this stuff knows customers will demand high resolution routing plans in easily exportable KML form. Wet sections still have a little protection, ie cable protection zones on navigation charts are more general.. But still sometimes ignored, ie accidental cable cuts from anchor drags. That's something where in Euroland, it's often relatively straightforward to identify ships in the area so cable operators know who to bill.
Same's true for some nefariousness. So if someone cuts a cable, that's usually quickly detected, then a cable maintenance ship will yoink the cable up, or look at it with an ROV. Unauthorised taps for a Tbps system should be detectable to the eyeballs on a cable ship.
Rest is kinda prelude to brown stuff heading for the whirly thing, ie all cables get cut, and satellites start going dark is likely to mean something more serious is about to happen than losing access to kitteh pics and hentai.