I do find the discussion of "sub-strategic" uses for clearly strategic weapons, such as Trident interesting. At least in the USA this has been discussed and generally dismissed, mainly because how would the enemy or others know that the launch was a tactical launch, and not the first shot in a strategic nuclear exchange? There were some thoughts to putting a conventional warhead on ICBM's, and you would in theory have the ability to put ordinace on any target in most of the the world in around 30 minutes.
Threat must be considered when looking at long range heavy strike/bombers. With the end of the cold war, I do not think the UK was looking at too many scenarios where heavier bombers were required. Heavy bombers are usefull for major peer conflict, and when long range, heavy load and endurance are required. How many of those do you expect to fight? Perhaps the threat and cost/benefit analysis didn't support a heavy bomber force. Such a force is very expensive to build and maintain, and tactical jets can likely get you an acceptable performance for most missions. The main thing you loose is long range capabilty when no airbases are close enough to the fight. So you loose strategic delivery (Cold war) and long range bomber strike (Falklands).
Yes the USA and Russia remain committed to the long range bomber. I see Russia as mainly staying in the game in attempts to remain relevant, and wave their sword. The USA does so as it remains committed to global pressence, being able to defeat any peer nation, and can afford to do so. (I won't go into the polictics of this being right or wrong). Any major conflict the USA is likely to be involved with will be far away from Stateside bases, and will require the range and performance of long range strike.
I think it is OK that the UK has stepped away from the heavy bomber. Perhaps they will find a time when the would have been usefull, but that is a risk that was calculated.