Thanks for the interjection SHJ
It depends what you mean by usable I guess.
Blue Danube bombs weren't that safe though not as scary as Yellow Sun Mk1's. The dates when more stable military grade weapons started to come on stream vary and this is due in large part to the secrecy surrounding this subject, but pre about '61 the RAF's 'independent' 'serviceable' A-bombs can't have been more than 25 Blue Danubes.
I don't think Thor was ever credible and was a US system.
At this time 61-62 it seems Red Beard tactical bombs were coming on stream as well as Yellow Sun Mk2s.
All the other atomic weapons available to the RAF were in effect US controlled. though officially designated as 'shared'.
My initial thoughts as to why there were so few British (if you like) bombs was because the rate of production of weapons grade material was so low.
But I now think what the record shows is that because of the '54 decision to embark on developing the H-Bomb brought on by the US Ivy Mike '52 and the Soviet Joe 4 '53 detonations, production of weapons slowed or stopped.
It is clear to me at least that the status of the UK as a Nuclear power was more spin than substance, of course the detonations of test devices were real enough, but I see them as largely symbolic ground thumping affairs.
My assessment is that it was the Sputnik launches that finally allowed the UK to get its hands on 'real' (military grade) weapons through co-operation with the US (though the Grapple tests may have played a role)
This is at odds with what the public were being told at the time and to a large extent what is commonly thought today, both about the real threat posed by the Soviets and the real capability of the UK of having its own independent deterrent.
Some free downloads that might be of interest.
Donald Mcintyre The Development of Britain's Megaton Warheads
Roy Dommett Blue Streak Weapon
1950 CIA Docs