It's standard practice to draw up war plans for every possible hypothetical situation. I have little doubt that Canada and Mexico both have "how to keep the US from curbstomping us" plans locked up in some government archive, even though there is no realistic chance of it being used.
Indeed, we did. I believe that it involved bicycle troops and waiting for the British to come and bail our asses out. If you can't tell, it was made sometime during the 1930s...
Which is convenient, since the American war plan involved eating Canada wholesale and waiting for the British, and the British notions of a war plan (since their government put the kibosh on actually formalizing any war plans against America for much of the interwar era) basically boiled down to sacrificing Canada and making peripheral attacks to wear the US down.
I am pretty sure those tridents you are talking of could hit things other than cities, I think I remember reading that they where designed to be able to take out hardened silos.
That may be possible, but that wasn't the intended use. The critical aspect of UK nuclear policy in the Cold War era was the "Moscow Criterion" - basically, their ability to destroy Moscow. In effect, since it could not be relied upon to equal the Soviet arsenal numerically and was independent of the American arsenal in operation, maintenance, and targeting, it had to be aimed against political and societal targets of destruction against softer targets in order to serve as an effective deterrent - that is to say, major cities