In fact they reacted before.

With spies inside Project Manhattan. Both the UK and USSR reacted with what they could, meaning their intelligence and espionage.

It's kind of a selection bias considering that the only country capable of building the Atomic Bomb was the US.

The UK provided key expertise to the Manhattan Project, so to say that they were "spying" is not fair. The UK bomb project had been going on for longer, and making it a joint US/UK project sped up development.

Members of the UK delegation were spies for the Soviets, and other members looked as hard as they could to find advantage for their own country. I certainly do not look askance upon the latter, and would have done the same.

I don't think anyone doubts the contributions our British friends made to the success of the project.