This isn’t exactly true. The British were well aware of the Manhattan Project- indeed, the whole thing began as the British Tube Alloys project before they ran out of money and convinced the Americans to take it on, with the British delegation taking prominent roles.

Britain also knew that the bomb was ready for use as the US didn’t have any bomb bay mounts strong enough to hold the weight, and they had to ask the RAF to supply a small number of mounts that had been designed for Barnes-Wallace’s 10,000kg earthquake bombs.

To be fair, the US thought that there were spies at Los Alamos, and they were right.

There's going to be a bit of contextual nuance when deciding whether to share intelligence and when. I'd bet if we had a respectable statesperson friendly to Ukraine's plight leading this country we would have gotten a few days notice at least.