\1 In real actual history, the US didn't developed a working nuclear weapon based on UK provided designs until after the German surrender.
So in real history there was no chance to "glass" an active EU war zone during WWII.
\2 In the GP's fantasy history in which the US left Europe alone to deal with the Nazi's on their own, the MAUD folk would have never passed on their research to the US and the US would have wallowed in their belief that nuclear weapons were impractical and continued on messing about with atomic power from big piles.
The US saw the MAUD report before entering the war. American scientists received copies as early as October (chemist James Conant received it october 3). But the US didnt commit to entering the war until after Pearl Harbor, December 8.
You are mixing the order of events.
Second you are overstating its importance. The primary contribution of the report is scientific feasibility.
Every part of the engineering and design of the bombs was done in secret in the US without any other country collaboration. They didnt need "UK provided designs" because there was no such thing.
MAUD made repeated paper approaches to the US to convince them of the feasibility of a weapons program, these were ignored.
Mark Oliphant made several advances and presentations in person before the US were convinced.
It's on record that regardless of any papers read, whether from the UK group, the Germans, the Japanese, et al. US scientists largely remained skeptical that a bomb was possible until repeated outside influence resulted in outside scientists baby walking them through the approach and the numbers.
> You are mixing the order of events.
* The German Instrument of Surrender was a legal document effecting the unconditional surrender of the remaining German armed forces to the Allies, ending World War II in Europe. It was signed at 22:43 CET on 8 May 1945
* Trinity was the first detonation of a nuclear weapon, conducted by the United States Army at 5:29 a.m. Mountain War Time (11:29:21 GMT) on July 16, 1945, as part of the Manhattan Project.
It would appear Germany did, as stated, surrender before the US had an actual working atomic bomb.
> Every part of the engineering and design of the bombs was done in secret in the US without any other country collaboration.
is straight up ahistorical; both the UK and Canada were deeply invovled in the project until, at least, the Atomic Energy Act of 1946. They played a crucial role in technique (magnetic seperation, etc.), raw material provision, provided a number of personal, etc.
It's a whole new level of naive if you think the Nazi's would have left the US alone after they were done with Europe.