I mean, the UK has 20+ fibre links to other lands. If one goes down, fine, if a second goes down, it's suspicious. If a third goes down, and there are Russian ships milling about over the location of the.. yes, there goes a fourth, it doesn't take long to realise what's going on.

Now, what the British Navy would do about this I'm not precisely sure. But even to escort the ships away would put a stop to it, and the UK wouldn't be cut off.