I think to get better clarity in your own mind you need to look at it slightly differently.
So first step, consider bank holidays and leave all in one leave pot. You are entitled to 50% of what a full time worker in the same role receives. So if they receive, let’s say 24 days plus 8 bank holidays, that is a pot of 32 days and it means you are entitled to 16 days, as is your colleague that works 50% same as you.
As a second step, now bring the bank holidays into it. Employers are allowed to tell staff when they can take their leave. So for the full time workers they are saying, you need to use up 8 days of your 32 day leave pot on the bank holidays.
For the part time workers, the employer can also tell them to use up some of the leave pot on bank holidays. So a PT worker that works Mon - Wed needs to use up some of their 16 days of leave (per my example) on bank holidays same as the full time workers do. A PT that does not work on Mondays does not need to allocate their leave to BHs because they don’t work that day anyway. So that person ends up with more freedom as to when to use their leave, but still gets the exact amount of leave as somebody that works Mondays.
I am a part time worker too, 60% and I don’t work Mondays. My leave entitlement for next year is 20 days (calculated as 60% of 25+8BHs). Only one of my working days next year falls on a bank holiday, Good Friday, so I have to use one of my 20 days for that and the other 19 I get to choose. On another year, I might only get to pick and choose say, 16 days because more bank holidays fall on my working days. That is why it needs to be worked out via the two step approach above, rather than as a %.