Britain is an island in the one sense that matters to a network engineer: almost every route out is a cable on the seabed.
Almost, but not quite. A fibre route runs through the Channel Tunnel, a sliver of trading traffic hops the Dover Strait by microwave, and satellite carries a little more. None of it comes close to national capacity — and the tunnel runs between the same two countries an attacker would already be working in. Parliament's own committee describes the UK as almost entirely reliant on subsea cable to reach the outside world.
Around 60 subsea systems land on these shores, and globally that class of infrastructure carries 95–99% of intercontinental data. In 2025 the Joint Committee on the National Security Strategy reported that it could no longer rule out UK infrastructure being targeted in a crisis — and that it was not confident the country could prevent such attacks or recover within an acceptable time.
The first wave is the login. Hardly anyone runs their own identity any more — we sign in through Google, Microsoft Entra, Okta, Apple — and every one of those depends on the overseas links. When they cannot be reached, live sessions run down and cannot be renewed, push-based multi-factor prompts stop arriving, and staff are locked out of systems that are otherwise running perfectly. A server in Slough is no use if the thing that says who you are sits in Virginia.
The second wave is the certificates. Under the CA/Browser Forum's agreed schedule the maximum life of a TLS certificate falls to 47 days by March 2029, with domain-validation data reuse cut to 10 days. Renewal also now requires corroboration from network vantage points outside the country. So an isolated Britain would begin losing TLS in roughly seven weeks — with no compliant way to renew.
That is the failure Dart Mail is designed for. Not the bang. The weeks afterwards.