I work 3 days, none of which is a Monday.
So, each time there is a BH, I get 3/5 of a full day's hours (for me it is 4.38hours) added on to my leave allowance. I can then take that back, when I want.
However, if - say Christmas day, Boxing Day and NY Day fall on my working days, then I can't work, and a full day's hours get taken from my annual leave.
So with my contract, it would be a bad idea to work Mondays, as there are 4 BHs that are always Mondays, alongside the 3 that move through the week (so potentially you could have 6 BH Mondays occasionally).
In your position I would work Tue - Fri, or you will be giving them hours from your AL.
Yours might need tweaking a bit though, as you are working more than 4/5 of the week, so I don't know how they calculate that.