There is of course the theory that a box of instruments was chucked into the area at the end of the build process, and that they were mounted where they landed...
Joking aside, the difference is mostly in a different crew concept. The US-built heavy bombers operated with a pilot and co-pilot, while the British-built ones used only a single pilot. This single pilot operation was supported by a flight engineer, who could take the controls for a spell and who would assist during take off and landing. This drove the different cockpit layouts that you describe.
As for when it changed, the next British-built bomber was the Canberra, and that was still single-pilot. The next one is the Vickers Valiant I think, and this one had a proper two-man cockpit.