The way my UK employer handles this is to add N extra days to your holiday/leave allowance, where N is 80% (or whatever % of fulltime you work) of the number of

The way my UK employer handles this is to add N extra days to your holiday/leave allowance, where N is 80% (or whatever % of fulltime you work) of the number of public holidays in a year; then they require you to book any public holidays which would be on your usual working days from your leave allowance. The net effect is that everybody gets the pro-rata fair number of extra days whatever their part-time schedule is, and nobody is in the office on a public holiday.