Russia nuking the UK is as likely as aliens invading. It just won't happen. The only realistic scenario in which a nuclear attack occurs is through an act of extremists who somehow gets their hands on the weapon, in which case any detterent means bugger all, they won't care about a potential retaliation, they'd probably welcome it.
The odds of Russia nuking the UK would be vastly higher as of now even before we factor in the Corbyn tribes' fantasy of trashing Trident.