My Two penn'orth:
Firstly, who would have dropped the bomb? I would think it would have been from a US 'plane, a B-29, rather than a Lanc. That would enable it to have remained firmly under US control, and it could have had a mixed crew for PR purposes (much as L Cheshire flew on 'Bock's Car''s mission). There would be no need for the US to make the UK a nuclear power (but this was before the treachery of many UK scientists was revealed, and relations in the nuclear world soured between UK and US - and for once, I think the US were quite right about this).
Secondly, the effect of the bomb on the city - Berlin is pretty flat, so a 20Kt blast, similar to that at Hiroshima and Nagasaki (and 20Kt is still the 'default' when considering NW effects) would have been pretty damned effective - BUT caused minimal fallout. An airburst results in very little, but causes maximum damage.
Thirdly, what was the wartime view of this in the UK? Well, NW had never been used. The almost religious hatred of them, which was stirred up and developed by the anti-nuclear movement in the '50s simply did not exist then (or for some years after - it appeared largely after the testing of the first H-bombs, and the death of some Japanese fishermen, but that's another story). There was no stigma attached, so why not use it? It's just a bigger bomb, and we were already using pretty big ones where required.
So, would we have used it? Damned right. It might also, of course, have had the added benefit of scaring the bejaysus out of Uncle Joe, and kept him well beyond the Elbe, thus making life a lot nicer post-war for millions of Europeans.