Yes, but consider the delivery systems. The UK has 4 SSBMs, and at best 3 of them are seaworthy at a time. Even if the UK managed to launch successfully with

Yes, but consider the delivery systems. The UK has 4 SSBMs, and at best 3 of them are seaworthy at a time. Even if the UK managed to launch successfully without those systems being taken down (and there will be a missile failure rate), Russia has sophisticated air defenses, and there will be a substantial loss rate. Only a small fraction of those warheads will hit. And those are not megaton warheads. These are small nuclear weapons meant for battlefield use. What's more, the UK is simply not going to go for a genocide; they're going to hit military assets, which are far more spread out. So while the UK can deter from conventional attacks on itself, it's by no means a MAD scenario. They do not really have a shield to extend across eastern europe the way the US does.