Your full time colleagues are entitled to 5.6 weeks, 28 days, holiday a year. It sounds as if your boss is letting them choose 20 of these and making them take the other 8 for the Bank Holidays, which she is allowed to do.
The 12.06% is simply the result of looking at an entire year for a full time employee - they will work for 46.4 weeks, have paid holiday for 5.6 weeks, and the ratio of paid holiday to work is 12.06% (5.6/46.4). They are paid for 52 weeks work, 5.6 weeks of which is paid holiday. You are entitled to the same ratio, so as Flowery says, for every hour you work you accrue 12.06% of an hour's holiday.
So each week you work for 32 hours, assuming you take no holiday in that week, you accrue 3.86 days holiday.
If you actually work (e.g. don't take any paid holidays) for 39 weeks in a year, you would then be entitled to 3239.1206 hours holiday (150.5 hours) at the end of the 39 weeks.
If, however, all your holidays are taken within those 39 weeks, then you be paid for 1248 hours, would actually work for 1113.6 hours and get paid holidays for 134.4 hours (134.4/1113.6 = 12.06%).
As long as you get that many hours, it's all legal. Your boss doesn't have to pay you for Bank Holidays on top of this entitlement.
strokes Flowery's hair.