Are you sure you get 28 days holiday pa (including Bank Holidays) for working 28 hours a week? That's quite generous ...
Assuming that's correct, going back to my previous example and putting the correct numbers in ...
* You work 4 days per week, so 4x52=208 days per year.
* Your holiday entitlement is 28 days per year
* Therefore, assuming no time off sick and no overtime, the number of days on which you would present yourself for work in order to receive your full salary should be 208-28=180 days.
* If you work more days than that then you're possibly entitled to more pay. It depends on your contract and on the circumstances in which you work more.
I think they are probably entitled to pay you for every day you take off, then ask that you take the whole lot out of your holiday allowance. I'd be very surprised if they didn't know their own rules. But, somewhere along the line, this has to catch up with them and you. There are only 180 days per year that you have to turn up for work. If they force more holidays on you now, then sometime later this year (assuming you haven't already done it earlier this year) you are going to need to turn up more than four days per week to make up for those missing days.