Though it was rather on the large size (I believe it was pretty much the max the Valiant could drop…). Britain needed to prove to the US that it could build a thermonuclear warhead, partly to gain access to the more developed US weapons. If memory serves, the demonstration drop was actually the big fission bomb, rather than the thermonuclear weapon, as they could at least guarantee it would work as planned…

It is correct, however, to suggest that a modern weapon in that yield range would pretty much have to be thermonuclear.