I think you are mixing two things. 

Many  organisations now pro-rata bank holidays. And add them to AL. So if you are a 0.5 FTE you get 0.5 of a FT bank/public holiday entitlement. 

Then if you were due to work on a day a BH falls you book it off from this entitlement. 

By far the most equitable approach is, as Keith says, to just add PHs to the holiday entitlement and pro rata the lot. This can mean that part-timers who work on Mondays feel that they are obliged to use more of their entitlement to take PHs when they might otherwise prefer to take it at other times of the year but, tbh, them's are the breaks.

It does, however, explain why my preferred days for PT working are Tuesday to Thursday, because then you actually end up with more flexibility to book time off.