Guernsey in the 70's, outward from St Peter Port IIRC. May not be unique.
I could certainly come up with other examples that are not on the UK mainland, but pay-as-you-leave was incredibly rare with UK bus operators. There were some very small-scale experiments in the 1950s and 1960s with seated conductors and payl, which all failed.
The difficulty of course was proving that people who were out to cheat the system boarded at one stop and not another, just the same trouble that railways have! The Guernsey example works because everyone gets on at the same place, or at least in the same fare zone, they use pay-as-you-enter in the other direction! It would also work if the route had flat fares, but otherwise pay-as-you-enter with occasional ticket inspections to deter over-riding has proved most effective over many years, once costs prevented conductors from being used.
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