I may not have followed correctly but it looks like some employers give part-time/shift workers bank holidays off paid on top of the minimum holiday allowance if they are contracted to work that day. The staff not contracted that day still get their legal time but are missing bonus time off
Well, they shouldn't be doing that. They're breaking the law.
If you work p/t, for every bank holiday they should give you the equivalent of the hours you would work IF your hours were spread over 5 days, for every BH.They should be added to your annual leave allowance.
Then they deduct from the allowance the hours you actually work when you take a day's leave or when a BH falls on a day you work. This is the simplest and most commonly used method.
There are other methods, but they are mind-bogglingly complicated.
Organisations like police, NHS etc when shift-working is long established will have their own methods. In the case of the PP's husband who works f/t but compressed to give a half-day on Fridays, when I've known people working that pattern, they've just had to suck up the fact that they get less time off when a BH falls on a Friday, but I'll check with DP when he's home if this complies with the relevant directive.
But everyone p/t who doesn't get added hours for BHs on non-working days is being robbed of 5-7 days per year (depending on when the Christmas BHs fall). PLEASE bring it up with your management of HR departments!
I don't work Mondays or Fridays and I get more than a week's hours added to my annual leave.