How might Russia respond to UK and US letting Ukraine hit it with their missiles?

Moscow has rattled its nuclear sabres, but experts say an increase in hybrid, ‘grey zone’ warfare is more likely

An image from a video posted by a Telegram channel affiliated with the Ukrainian military on Tuesday claiming it shows US-supplied ATACMS missiles being fired from an undisclosed location in Ukraine. Picture: Lachen Pyshe Telegram via AP

Moscow has threatened to retaliate for the decision taken by the US and the UK to allow their long-range missiles to be used in strikes inside Russian territory, and warned nothing was off the table. 

Earlier this week, the Kremlin announced a formal change in its nuclear doctrine which specifically envisages a possible nuclear response to Nato-assisted strikes on Russian soil. So, how far is Vladimir Putin prepared to go?