The employer can fix up the imbalance -- if there are N bank holidays in the year, give all 80% part time workers 0.8N extra days of holiday allowance above their usual amount (etc), and mandate that they cover the bank holidays with days from their allowance if they would normally work them. (My UK employer does this.)
It’s time and a half and a day in lieu for working a stat where I am. If you work it much outside 8.30-5 penal rates rise, but that part is employer specific rather than legislated.
