Phil Lo Bao is a substantial aviation author on airlines of the period, such as his history of BEA. I seem to recall he mentions in his books and magazine articles that his father was one of the management at Southampton during this period, and many of his published recollections, and possibly photo collection, go back to that time. You may be able to make contact.
Regarding that terminal building, was it opened in 1967 when the adjacent railway station was opened ? I used to look from the train which stopped there and think that it must have been the closest station platform-to-terminal distance anywhere.