The Sedan land-mine is about all you could use one for at that stage of war.
IIRC, at first, no-one had aircraft capable of delivering the physical tonnage of a gun-type bomb, never mind as one great lump. It was the four-engined Lancaster --Spun from the twin-engined Manchester ??-- then stretched and up-up-engined that eventually proved capable of hoiking the Tallboy and other 'traditional' big bombs to their targets in Europe. What was the state of play of the B-17 at the start of WW2 ?? Never mind the silver B-s that eventually did for Japan...
Assume they're all stuck with gun-type bombs. One logical target would be the Tirpitz & co in the fiords. You'd need X-Class mini-subs a bit sooner, but those could ferry a nuke, and it would NOT need to be up against the ship's hull per the OTL attacks. Outside the immediate 'torpedo' netting would serve...
By the same token, had the Germans got the tech first, their logical target, beyond the Thames & London, would surely be Scapa Flow. There was a submarine attack, but a nuclear mine would cause devastation among the un-prepared ships. It might not sink many, but its shock-wave would surely cripple the anchored fleet by damaging boilers, piping, turbines, bearings etc etc. and injuring crew...