Yes, there is fiber infrastructure in the Channel Tunnel [0]. I'm pretty sure that while any one good link is vastly better than zero links, no one link is sufficient to carry all traffic from/to the British Isles?
ok, so a quick check of submarine cable maps shows around two dozen from England across the channel (ignoring those to the Nordics and North America)
Would a pair of cable sets replace all of that? That's a lot of data, routing, and redundancy gone. Sure, if they had to try to make it work, they would.
But you seem to be suggesting that because there's a couple of cables in the tunnel, it's OK if the undersea links all get cut by RU/China? If so, WTAF?; you need to explain that
[0] https://www.colt.net/resources/colt-successfully-completes-t...