On the flip side, if the restaurant just upped costs by 10% they would have to pay increased NI rates, increased pension costs. Realistically the extra "profit" would just go to the owners/shareholders.
Not necessarily, if it is a mandatory service charge applied to the bill that OP is talking about then NI, pension etc... deductions still apply, that's different to say handing the waiter a discretionary tip.
Tips at work
Tips at work do not count towards the National Minimum Wage - tips and tax, what your employer has to do, advice and help, cash in hand payments
www.gov.uk
Service charges
These are added to the bill before it’s given to the customer.
If the charge is compulsory, it’s not a tip so if your employer gives it to you, it’s treated in the same way as your wages.
If it’s voluntary, you pay tax and National Insurance in the same way as for tips.