Part-time staff get their holiday allowance pro-rata’d, this includes bank holidays. If the staff member normally works on a bank holiday then they need to book that day as annual leave.
The staff member could request (by speaking with line manager) to move their working week round on the week of the bank holiday so that they work on a different day instead, and not use annual leave. E.g. If usually work Monday -Wednesday on the week of the bank holiday they could work Tuesday-Thursday.