Groves was asked, post war, about the possibility of using 'the bomb' against Germany.
He said that Roosevelt, who was very disturbed by the 'Battle of the Bulge' asked him about the possibility around this time. Groves gave the President a list of reasons why it would be a difficult, though not impossible, task. Not least was the absence of any B-29s in Europe, and the inability to camouflage the 'specials' among any significant number of the type.
Groves concluded.
"The bombing of Germany with atomic bombs was, I would say, never seriously considered to the extent of making definite plans but on this occasion I told the President, Mr. Roosevelt, why it would be very unfortunate from my standpoint, I added that of course if the President — if the war demanded it and the President so desired, we would bomb Germany and I was so certain personally that the war in Europe would be over before we would be ready that you might say I didn't give it too much consideration."
As far as the British attitude can be discerned, and they would have had to approve the bomb's use, the possibility of using nuclear weapons against Germany, as a deterrent to the V-2 attacks, was discussed with Churchill in August 1944. I don't believe the British would have had any objections, on principle, to its use against Germany.
Cheers
Steve