Panic, no, but War Plan Red on how to fight a war against the UK and its various options like Crimson (Canada) for the dominions had existed for decades. Red more or less boiled down to keeping the British Fleet away from Canada and the US East Coast, blockading, and ultimately stalemating. While that plan was pretty much mothballed in 1939, one concern that did arise shortly thereafter was that had the British wound up in some sort of accommodation with the German government, the United States might have to make a preventative strike on the British Fleet much like the British did to the French Fleet at Mers el-Kabir in 1940. This didn't get much past talk, but Churchill did reportedly get rather annoyed when at one point in some negotiations American war planners dared to mention circumstances that would involve scuttling the British fleet upon the event of British surrender.

To your what-if question about potential focus shifting to North Africa, though, there actually was a lot of time spent in early 1941 discussing the Azores, which actually got a full work up in War Plan Gray. In the late spring, a potential German move on Dakar was an even more major concern, with staff looking at strengthening Brazil as one option. This never got gamed up into a War Plan proper, but it did reach the highest levels of the government. From Conn and Fairchild's The Framework of Hemisphere Defense:

No matter what else was done, both Secretary Stimson and the Army General Staff also wanted to move a small security force (about 9,300 troops and 43 planes) to northeastern Brazil as soon as possible. On 17 June General Marshall pointed out to Under Secretary Welles that, as of 10 June, there was not a single American naval vessel within 1,000 miles of the eastern tip of Brazil, and no United States Army forces within twice that distance. In an estimate submitted to General Marshall on 18 June, G-2 expressed its belief that the German push southwestward had reached ominous proportions: ten thousand Germans were believed to be in Spain; it was "reliably reported" that the Germans had concentrated transports in southern French ports ready to move four divisions to Portugal; German artillerymen, equivalent in strength to two regiments, were believed to have moved into Spanish Morocco; and G-2 was certain that German submarines were being supplied from the Canaries, and probably from French West African ports as well. If this G-2 estimate were anywhere near accurate, it certainly behooved the United States to take some sort of quick action to protect the Brazilian bulge. This was the view presented by Secretary Stimson and General Marshall to the President in a bedside conference on 19 June, and the President told them he would direct the Department of State to find ways and means of getting American troops into Brazil.

This would have presented immense logistical problems as there simply wasn't enough transport available - while CINCLANT, King had pushed back hard against the Army when presented with the even higher shipping requirements for the Azores - let alone scrounging up the men or munitions, but it was something that was getting looked at very hard in late May and early June 1941.

As it turned out the subsequent invasion of the Soviet Union a few days later made all of this moot. However, had the UK fallen, its likely something similar would have been implemented, albeit on a much larger scale.