Do you have to work a bank holiday first before you can take a day of annual leave?

Bank holidays are not supposed to be linked to a person's standard annual leave allowance.

Bank holidays are fixed dates which rarely last more than three consecutive days including their associated weekend, and these are not always the days when you want to be away. So most employers have a staff leave policy which allows maximum use of bank holidays whilst making sure that the workplace is not empty between them.

But most employers give their staff a "leave year" which is a 12 month period within which your 28 days must be taken. The leave year may be allocated according your name or month of joining, they are hardly ever related to the calendar year.

Some employers allow their staff to "borrow" a day from their next Leave Year if this helps them to take a particular holiday and they've used up their current allowance. But that policy varies from company to company, and yours may have a policy which does mean that ALL holiday, including BHs must be used up before you can "borrow". If your company has a policy like this then it is legal.

If you have used up all your 28 days before you have finished your "leave year" then you cannot take any more leave until you have entered your next Leave Year. If this leaves you with the set of 8 BHs before your current leave year ends then that is all you are entitled to take.

A lot of companies have policies which enable them to ration an employee's leave so that the workplace is not left empty because everyone's gone away at the same time. But that is nothing to do with BHs (or shouldn't be).

If your employers are really making you work bank holidays as a qualification for starting your annual leave allowance then that would be illegal, and they would face some rather large fines.

This is why it is more likely that someone has not properly explained the concept of your "leave year" to you.

But if it really is the way that you describe then you must report them to your union if you have one, or an Industrial Tribunal (look up local tribunal contact in google or ask at your local Citizen's Advice Bureau).

If I understand this right, you already used your 20 days, and you plan to work on a holiday in the future, so you want a day off now to offset it.

I see no reason why your employer must do this. You aren't entitled to that holiday yet because it hasn't come up on the calendar yet. For all your employer knows, you could quit or drop dead before then.

Unless your contract explicitly says you can, of course. But if it did, you probably wouldn't be asking the question.