They did that because they needed a reason for their forces to be near the border (Of course it wouldn't fool a concussed chipmunk, but that's not relevant). This is why historically Russian military exercises often get up NATO's nose, because Russia does this a lot, it claims " training exercises" as an excuse for its forces to be in NATO waters.
But simulating a nuclear strike doesn't require forces to be in NATO territory or anywhere near the UK.
It's not the exercises itself that's the problem, it's the fact that Russia often uses them as justification for moving forces around.
If Russia simulates a nuclear strike against London for example, how will anyone in London even notice. They're just going to say they did it. It won't actually involve doing anything.