In the 1950s, 60s and much of the 70s, the only commercially available insect live food in Britain was the mealworm, imported from Holland. Even then supplies could be spasmodic. They were also expensive and so most reptile keepers kept a culture. Earthworms were the great natural standby. A few people obtained locusts and bred them but they were not available commercially.
While reading through old copies of the Aquarist, I came across this advertisement from Xenopus Ltd. Crickets arrived in June 1976.