Its interesting to note also, before the "its only a late war weapon" crowd emerges, that it would have been built far earlier if the British Govt had had faith in it designer. He actually proposed the Grandslam well before he had even thought of the bouncing boms used in the dam-busting raids. Nobody thought it would work. Only after he proved in that raid the a bomb detonated under the surface (be it solid or liquid) would have greater force than one detonating in the atmosphere did the War Ministry give the idea serious consideration. And even then they built the half-sized Tallboy as a testbed.
It probably could have entered service at least 18 months sooner if they had given the go-ahead.