If your contract is 20 days choosable and 8 bank holidays, then you are only entitled to be paid for 8 bank holiday days, regardless if there are 9,10 or more in the year. The important thing is that the total number of paid days off you get is 28.
Are you saying that they are giving you less than 28 days paid leave in the year? If so, they are incorrect.
If you are saying that you have 20 days choosable, have already had 8 days bank holiday paid, and the Tuesday is a bank holiday, so you should have it off paid, then you are wrong, as that would add up to 29 days paid leave, and you are only entitled to 28 as per your contract.
You are only entitled to 8 days bank holiday paid, not all bank holidays. In England, most years 8 days will equal all the bank holidays, but this year there was the queen's funeral (plus it seems that you have an extra bank holiday in Scotland).
In short, if your contract specifies that you are paid for 8 bank holidays, you are not entitled to be paid for any further bank holidays over that amount. It's not the designation of "bank holiday" that matters, it is the number of paid bank holidays that you are entitled to and have already taken that matters.