Why would the UK not retaliate? If the UK is destroyed the thirst for vengeance would be insurmountable. A nuclear attack on the UK wouldn’t be limited to the UK either, they would have to nuke the rest of NATO as well to try and take out as much nukes as possible pre emptively, they couldn’t risk a NATO nuclear retaliation being launched before they had chance to pre empt them. Even if the US and NATO held off, the UK nuclear response would leave what was left of Russia devastated and unable to defend itself against a largely intact West.
Even Obama said we could absorb a nuclear attack
Our leaders do not have the will to use nuclear weapons