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Firstly tag your employees by country, so I would give them an "England" tag for those getting English bank holidays and "Scotland" for those getting Scottish holidays etc.
Set up a leave type of public holidays, you can give it a meeting type of public holidays too.
create an allocation request for the full number of holidays in the year and leave type public holidays, set the allocation mode to be by tag and choose your "England" employee tag to apply it to.
now for each day that is a bank holiday create a leave request, again do it by employee tag so they all get the holiday together. Do this for all the holidays in the year and that should use up the allocation in full.