"The practice of tipping began in Tudor England. In medieval times, tipping was a master-serf custom wherein a servant would receive extra money for having performed superbly well. By the 17th century, it was expected that overnight guests to private homes would provide sums of money, known as vails, to the host's servants. Soon afterwards, customers began tipping in London coffeehouses and other commercial establishments".

https://en.m.wikipedia.org/wiki/Gratuity

Tipping in the UK at restaurants has been a thing for a long time.

However it’s normally included in the bill as service charge so you just pay the bill and don’t have to decide how much to leave yourself. In London it’s usually 12.5%.

You are allowed to ask them to remove service charge from the bill but it’s an uncommon thing to ask.

I just treat the service charge as part of the price (when deciding where to eat or what to order) and would only not pay if it was really awful (not happened yet) but I will tip a bit extra if service is really good.

Gratuity charges are what they mean. Parties over a certain size have them added, or it’s sometimes offered on the payment terminals.

Always a fixed percentage of the bill.

It's extremely common for there to be a (not really) optional service charge included where there is table service.

I suppose even if it's paid 98% of the time, those 2% could still serve a signalling system where customers receive truly terrible service.

Since the Tudors, before the US as we know it.