The Swiss must have strange rules for under 5s.
Yes I don't think we can use Swiss expectations of boarding at a young age to judge UK schools. The Swiss expect reception age day school children to walk to and from school, alone, twice a day (home for lunch). A British friend who lives there walked her son to school on the first day and the teachers said "thanks for that, we don't want to see you doing that ever again" even though the walk was at least 20 mins and crossing roads. There was also an assumption that she would be at home waiting for her son with lunch for him every day for two hours in the middle of the day, plus school is a half day every Weds.
One can imagine that a Swiss single parent commuting and travelling while working for an international company, when faced with these expectations from Swiss day schools, might start googling boarding schools for 3 year olds.