I dont think old editions are available on the IEE website, or elsewhere for that matter.

I have a copy of the original 14th edition, published in 1966 ("Copies may be obtained from the institution, price 17s. 6d. including postage" [Linked Image]) and the amended 14th edition converted to metric published 1970 (it had gone up to 25 shillings by then!).

I don't have any earlier editions of the Regs. themselves, unfortunately, but I do have a copy of the A.S.E.E. (Association of Supervising Electrical Engineers) guide to the 13th Edition, 1955, as amended up to December 1963.

By the way, the voltage classes were still the same back then:

Extra-low: As described above.

Low voltage: Exceeding extra-low but not exceeding 250V, either between conductors or to earth

Medium voltage: Over 250 but not exceeding 650V.

High voltage: Over 650V.