This is off the top of my head, but IIRC the American rockets were initially developed by modifying the body of a naval gun warhead to mount on a rocket, while the British 3.5" rocket had different development roots, and consequently had a more powerfull warhead.
But yes, based on the 1k bomb, the 3.5" is equivelant to 140lbs of ord, while the 4.5" is only 93lbs.