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

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 that nothing is 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?