Back in a day service charge was invented by top tier restaurants to assure their waiters and barstaff, often helped by busboys and barmates, were properly tipped and rewarded for their work in the days when no minimum wage existed. The problem was two fold - on one hand, business model relied on waiters and barmans going out of their way to serve customer to the best of their abilities, and thus being tipped according to the service provided rather than paid equally by employer and thus discouraged from going out of their way to provide spectacular levels of customer care. On the other hand, around 80ies when credit cards became preferred method of payment and business expenses started to rely on printed invoicing, service and tip charges could no longer be included as part of "representation costs" charged by yappies with expenses accounts to the mothership, and the old custom of tipping in top tier restaurants simply died. Especially during lunch service.
With introduction of minimum wage and EU employment laws, service charge became a way for the restaurant to cleverly offset service and staff costs in relatively ruthless way. The method relies on the fact that clients paying £20 for three pieces of asparagus circled by a drip of malt vinegar and olive oil will be too embarrassed to request such charge to be removed from their bill in front of their peers.
One should be aware however, that service charge, in most cases, does not go to the waiter serving your table. Instead, in 99% of cases it is taken by the employer, taxed by inland revenue (unlike cash tip), covers the cost of laundry, written off cutlery/dishes/glasses during service, often expiry date food and wine losses, and then is divided among staff as part of staff wage according to some sort of internal distribution scheme or duty share. This method is called tronc points or tronc scheme, and is
recognised by inland revenue as legal method of payment (as in base wage + tronc to create minimum wage).
Service charge used as service cost + tronc scheme is a horrible and scoundrel way of effectively offloading staff and floor service cost - anything that happens on the floor in terms of human force and hardware is no longer paid from owners/employers profits and instead is practically forced from customers pocket as an extra/off the menu charge under pretence of tip and then stolen from waiters/barman pocket.
The only way to end this, is to make it "dirty". A bad thing for restaurant to add to bill. Talk about it, write about it. Make it into party anecdote - call manager over to the table and ask whether the service charge goes to the service staff or whether it also pays for cutlery, table cloths and napkins before waiter is being paid and then "boo" them out for being scrooges. If it goes towards anything else than your waiters wage, ALWAYS request the charge (often as high as 13-20%) to be taken off your bill (if asked for reason just say the menu states your meal costs XX amount and that's the amount you are willing to pay for the meal) and then if service was indeed worth rewarding - TIP YOUR WAITER INSTEAD. That way employer will have to pay for the service costs and minimum wage just like any other business and tip will be tip on top of your waiters wage, instead of being part of their minimum wage.