How could anyone have believed the UK would

I never quite understood this either.

A forced treaty might say this, but I'd expect that "mutineers" (secret orders) would take the fleet to a safer area like Canada or the US, where they could still be aimed at the US.
 
(OTL, during parts of 1940, COs of warships were given lots of cash before putting to sea, in case of invasion before they returned, so they could purchase fuel to travel to N America. If, however, Churchill had been replaced and new orders demanded they return to hand themselves into captivity, it would've depended very much on the conscience of the individual COs as to whether they did this.)
 
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