I pay the waiter or waitress any tip in cash. However, that means I have to trust them to act properly with it.I'll cross out the 'optional service charge' which is referred to on the bill.
Hotels and restaurants may run a tronc.The staff are meant to pool all their tips. The whole lot is then divided up and shared. The idea is that staff who are not high-profile, not the ones who are in the front of those who deal with you face to face, get a share of the tips. Such people as chefs may be included in this.The system also pays seniors more than juniors. If it were otherwise, some 17 year old junior who opens most of the car doors would get a tip from every driver and be getting more than the senior man who may open few but who is in charge of the forecourt. I had a student who did this junior job at the Savoy Hotel once. He was taking hundreds of pounds in tips. His superior was taken a tiny fraction of that, in person. However, once the total was shared out the senior man took the biggest cut, and junior didn't end up on a manager's wages

The system has had its critics, because dishonourable seniors may abuse it, but places like The Savoy have long experience and a tight system to stop abuses.
The French have it cracked. No tips, except very occasional small ones of a few cents or the odd euro left in the change tray, are given in restaurants.That's because the law is set against 'service charges'. The price you see on the menu outside is the price you'll pay, without any 'extras' for 'service' or 'taxes'.
By the way, a tip of 20 per cent in Britain would be thought very generous. Ten, twelve,even fifteen maybe, but not twenty.