It would be really bad for a small group of people and would save the lives of hundreds of thousands of others. Say Putin does strike Britain. A bunch of people die in the initial strike. Then Britain, America, Poland, Germany, Ukraine, France, Canada, and probably a handful of others all collectively turn the Kremlin into glass and dust in a retaliatory strike. China and North Korea both rattle their sabers about it but neither of them wants to throw down with the Allies 2.0 because it doesn’t matter who you are, going to outright war with a world cooperative force is a capital-letters Bad Idea.
Otherwise Putin continues to slowly grind his way across Eurasia for the next decade while every other country’s politicians hem and haw about who they want to help and how much. Meanwhile thousands of people die daily as the Russian meat wave slides its way down toward the central EU.
There’s a very good reason that Putin has been performing military ops in Ukraine and only vaguely threatening everyone else that got involved. He knows that if he manages to actually draw the ire of the full Allied military might the remaining lifespan of his army and capitol will be measured in minutes.
If this were just a straight up shooting war it would have been done and dusted two years ago. Instead it’s a political war, and that’s stopping a bunch of politicians from actually making a move on it. Nobody wants to be the guy that started World War 3, and Putin is leveraging that to enormous effect.