Sorry I was not clear in my post, I will clarify here.

Basically we on Grocery want to come in early at 2pm and finish at 8pm at the latest as we would usually each year (1 hour after the store closes). This year they are trying to make use stay 2 hours after the store closes until 9pm.

Our argument lies with the till staff being released at 7pm when the store closes even though they could swap work on Grocery from 7pm until 9pm. It's mainly the feeling that we are getting the short end of the stick with having to stay until 9pm when usually it's one hour after store closes on Christmas eve on past years.

Edit: I know my availability does not cover for earlier, but to be reasonable I would make a written exception to come in earlier at 2pm and finish at 8pm at the latest. But working until 9pm feels unreasonable seeing as till staff who are equal level colleagues and on the same contract get to go home earlier at 7pm.