I only did one sortie over Russia on 543 in 1964 when we spent nearly two hours over the Kola Peninsula. The 'source' said that a nuclear sub - can't remember the name- was being readied and its departure date was given. We night stopped at Kinloss, and set off early the next morning and got the Valiant up to about FL530, and we were sending the results using morse to, I think, a USN P2 operating out of Bodø.
What surprised me was, even as a 20 year old, that there was absolutely no briefing whatsoever about what to do if intercepted, what if we were forced to land, escape and evasion, how to deal with interrogation etc etc. One of the crew's job was to get up on the Plotter's table every so often to look through the periscope to see if we were contrailing and if we were, to descend or try and climb!