[Rainbow Six]

It's not impossible that the UK bases were hit with sub 1mt weapons, but certainly in the case of Portsmouth I think it's unlikely given the fact that it is the new UK Capital and is referred to on multiple occasions in the Survivor's Guide to the United Kingdom.

I’m no naval expert either, but I'm inclined to accept Webstral's theory that the RN had already suffered such heavy losses that the Soviets no longer viewed it as a threat to be be the most plausible explanation - there’s the quote about the last major naval fleet in the World being shattered by June 1997. Either that or they had already suffered heavy enough conventional damage to make nuking them unneccessary.

(Probably noteworthy that that quote specifically says “in the World”. One could use that as an argument in favour of the French Navy also having suffered significant losses, but it would then have to be extended to include all Navies. But we know that even after that point the French retain the capability to move a large force to the Middle East (and also combat troops to Canada, per Challenge #30), and patrol the English Channel and elsewhere - there are other references to the French Navy here and there, chiefly (I think) in the Last Submarine trilogy, e.g. page 48 of Med Cruise, which states “The French have come to view the Western Mediterranean as their own private Sea”.)