Well the BSI still sell copies of the standard, and I am pretty sure you could get a copy from a library.
I do not have the standard itself, but have some extracts in another publication.
CP3, Chapter V (1952)
Office floors above the entrance floor, floors of light workrooms without storage. Minimum imposed loads 50 lb/sq.ft.
Office entrance floors and floors below entrance level, floors of classrooms. Minimum imposed loads 60 lb/sq.ft.
There are additional alternative minimums but they are a dependent on spans, arrangement, construction type and are more tricky to convey in a web forum.