Service charge

I am doing my wedding on a budget as I have planned in the space of 7 weeks due to the ill health of a family member. We are having our reception in a pub/restaurant which we've paid £500 for the venue hire and the have a minimum spend of £3500. When giving our final numbers for food, they have added a 12.5% service charge which amounts to nearly £300 which will come out of our minimum spend. I don't really feel comfortable paying that much when they have already made flat out £500 before we've even booked any food or drinks. Am I being stingy in saying I would rather offer the staff a tip on the day and waiver the 12.5% service charge that they have requested upfront or would you be happy to pay that?