Didn't pay an "optional service charge" in a pre-booked London restaurant, did I do the wrong thing?

It was booked as part of a theatre deal, meaning we had already paid for our meal beforehand. Usually I'll tip but they were adamant we paid a 拢2.50 service charge - listed as optional - and I refused on the principle that they demanded money to which I was given looks of revulsion. Should I have paid up?|||If it is optional it is your decision. this is not USA - here staff are paid a minimum wage.