So, you have to have 20 days plus BHs. In your case they're shorter days.20 + 8 = the minimum legal holiday for a full time worker.
I'm going to assume you work the same hours every day (6.25). If not this won't work.
But if you don't then in days, you should have the same as someone working 4 days a week, but each day you're paid for should be shorter.
So 28*0.8 = 22.4
For a normal year at least 4 will be on a Monday (Easter, early May, late May, August).
So that would take you down to 18.4
This year Christmas, Boxing Day and NYE also fall on your working days so that taks it down to 15.4 with the remaining one being good Friday. Do you also work Fridays? That's the only way I can get it down to 14.4
If full timers get a specific amount of leave that's higher than that, you/I could recalculate using an amount higher than statutory.
In terms of using that remaining leave most effectively, could you e.g. take a summer holidays week off on the week of the BH (3 days) and also the summer half term which usually contains the late may BH (3 days), plus Christmas to new year (2 days). That would get you 3 full weeks off plus an extra day for 8 days holiday so you'd still have 1.5 weeks left. You could also do the same thing one week either side of Easter. You might not get all of them, but definitely worth focussing on those weeks in the school holidays (assuming you have school age kids), or any BH weeks if not.