Not really.
Let's hypothetically assume Russia decided to drop a single nuke on Reading to teach us a lesson. About 500,000 people are killed or maimed. The whole world breathes in and says "Hell, we must de-escalate. It could be Seattle or Turin or Lyons next." (Humans are nimby's remember.)
Putin is allowed to finish up with Ukraine so long as he calls it a day on his nuclear strikes and the war is declared over. There then follows a long, cold, icy war. Article 5, or whatever, was never executed, but billions stay alive and the world goes on. They say "Poor Reading, but you know, had we executed Article 5, Seattle, Turin and Lyons would have gone also and with them 3,000,000 souls."
The NATO protocols are not prescriptive enough. In any event, nothing ever happens until it happens. So, don't bank on it.
Thank your God the UK has nuclear weapons. Just one of the UK submarines carrying the trident deterrent could destroy most major Russian cities in a single launch. And because one of those is at sea at any one time - and I am sure this number may increase soon - this is the only deterrent against a nuclear attack on the UK. Russia knows this.
We will shortly need to replace trident. The cost will be about £200 billion. That is equal to about 90 days of HMRC tax revenue. Sell of some Russian owned Park Lane property and that bill gets cut by about a quarter.
Ukraine and Belarus gave up their nuclear weapons. Enough said.