Of course tipping in the UK is a 'thing'. Of course it isn't the law either, but it doesn't have to be law to be the normal thing to do. And yes it may be "ridiculous" but it doesn't mean it doesn't happen.
Tripadvisor: "In restaurants where you place your order with your waiter/waitress and receive food, and your bill, at your table, it is usual to tip around 10%. The expectation does vary from place to place - in fine dining restaurants where you receive personal service, a tip would always be expected (while never compulsory, it would be considered rude unless there was a problem with the service), whereas in the most casual of restaurants tipping is not universal."
The Independent: "Normal practice is to tip the waiter 10 per cent to 15 per cent of the total bill. If a service charge is included on the bill, it is rare for a customer to tip twice"
Daily Mail: "Research reveals that 87 per cent of Britons always leave a tip, contributing 9 per cent of their bill and tipping, on average, £4.18"
VisitLondon.Com "It is customary to leave 10-15% of the bill when eating out. However, restaurants often add on a service charge (usually 12.5%), especially if you're in a large group, so it's worth checking your bill if you don't want to tip twice. It's not customary to pay a tip for fast food, self-service or takeaway meals"
Lonely Planet: "Tipping: Restaurants Around 10% in restaurants and teahouses with table service. Nearer 15% at smarter restaurants. Tips may be added to your bill as a 'service charge'. However, paying tips or the service charge is not compulsory"