I'm not saying the United States didn't field multiple bombers, I'm saying they were differentiated. The British developed four strategic bombers with similar performance to do the same role and fielded three of them. The United States tested two strategic bombers and chose one (Boeing B-29 vs. Consolidated B-32 for the first one, Northrop YB-35/YB-49 vs. Convair B-36 for the second one, and Convair YB-60 vs. Boeing B-52 for the next one) except during World War II. There were other bombers in the USAF and USN as well, but they had different performance and roles.
I just don't see why there would ever be a need to develop and field more than two systems of the same generation for the same role.
The v bomber specification, design, production period lasted 20 years (Tizard committee in 1945, specs were written 1946 to 1948). In that time, the us went through several generations of bombers. you might say the Sperrin was the 1st generation, Valiant the 2nd, and Vulcan/Victor the 3rd.
If there had been a war the Sperrin would have been flying and fighting in about 1950 (let's not forget it was cancelled in 1949)... 16 years before the last V bombers were delivered.
The valiant did fight in Suez in 1956, when the Vulcan and Victor were still far from combat ready.
Plus there is one more thing. when the British specified the aircraft, the ministry of supply would give no guarantees as to the eventual dimensions of the British special bomb. Aircraft manufacturers were told they might be expected that they might have to redesign the aircraft significantly to accommodate it, once the bomb was ready - so there was a huge amount of uncertainty.