I'm not really old enough to remember but I don't imagine it was a sudden change in policy from the British Retail Consortium or whoever. Early closing day did get a mention in The Likely Lads film in 1976 when Bob and Terry are considering a trip to Boots the Chemist. The supermarkets would have really been gathering pace by then and I don't think they ever closed on Tuesday or Wednesday afternoons. I expect it was a gradual change over some years, perhaps even more so in rural communities.