Ancientgran is wrong, you do need to book the full day as the bank holiday.
This is why the general advice to people working part time is not to work Mondays if you can choose - because the majority of Mondays fall on a bank holiday.
Example
Full timer (works full days Mon-Fri) gets 20 days holiday plus 8 bank holidays (in a normal year). Let’s say full time is 35 hours per week (7 hours per day). Their holiday entitlement is 28 days or 196 hours. Each bank holiday they use 1 day or 7 hours of their entitlement. This means they have 20 days/140 hours that they can choose when to take, and 8 days/56 hours which are taken on bank holidays.
You work 50% of the time so are entitled to 50% of the hours. You therefore get 10 days/ 70 hours as 50% of the standard holiday, and 4 days/28 hours for the bank holidays. Each day of holiday taken, including bank holidays, uses up 7 hours as that’s the normal working day.
It then depends which bank holidays fall on your working days. In any year there will be at least 4 that fall on a Monday, 1 on a Friday then the Xmas, Boxing Day and NYD ones vary.
So people who work Mondays may find they have up to 6 bank holidays that fall on Mondays depending when Xmas falls. These people then have to eat in to their standard holiday entitlement to cover the bank holidays. They still get 50% of the days off of the full timer, but slightly less choice when to take some of the days.
Those who work Tue-We’d are much better off because at the most they will have 3 bank holidays that fall on their days, so my part timer example with 4 days bank holiday entitlement has an extra day that they can choose when to take. Same amount of days/hours off as the Monday worker, just more choice when to use it.
This year is a bit different because as pp have said there’s an extra bank holiday and one of the Monday bank hold has been moved to a Thursday, but the method of calculating it still stands.
Hope that makes sense!