Restaurants are treated slightly differently to retailers in this point of law. The invitation to treat becomes a binding contract once you have placed your order and it has been accepted by the restaurant. Service charge does not form part of either the invitation to treat or the contract because of its discretionary nature. You are entitled to remove it as it is not part of the contract in law. If you have a truly terrible meal and you have good grounds to believe this you may also be entitled to reduce the payment to the restaurant accordingly. I’m not sure if this is still lawful but it used to be the case in England.
Although Restaurants my be treated slightly differently I cannot see how the contract would be binding once you have placed your order and the restaurant has accepted it, if you still had the ability to change the amount payable because you had a bad meal, albeit that if you complained at the time the food was served to you, and you had not eaten it fully, you would still have the right to refuse or amend payment. Once the meal was eaten fully you would have no right to pay less even if you didn't enjoy the meal.
If however the Restaurateur agreed on a lower charge, and you agreed, then this could void the first contract and form a new one.
One thing that does puzzle me, is most establishments nowadays take, and in some instances prefer their customers to pay by card, rather than in cash, and this would normally mean the business would be charged a fee to process the card payment, a fee of about 2% (guesstimate) of the total, and I often wonder if this extra fee would come out of the service charge, before the balance was paid to the staff,.as charging extra for using a debit/credit card to pay for a purchase is illegal,
After these excess payments were banned for using cards,( and businesses were allowed to charge up to 20% for using a credit card) most retailers and restraurants started putting a service charge on the bill, and as it didn't stipulate it was for using a card for payment it was thus legal.. Just Eat for instance have a 50p service charge on an order, which used to be only charged if you paid by card.
One thing a lot of sellers don't realise, if someone pays via Paypal, Apple pay or by credit or debit card, it is illegal to charge an extra fee, and quite often you will see items priced plus 4% if paying via paypal. Normally it would be better to price the article including fees , postage etc.
So maybe this is one of the reasons service charges are now quite common.