Oh really? First hit from Google for “guide to uk tipping” is trip advisor (the biggest online travel guide);
Tipping is not always expected in the UK in the way it is in the United States or Canada. All staff in the UK, must by law, be paid at least the National Minimum Wage (£6.50/hr (2014 rate, lower rate applies to those under 21 years of age) whether they receive tips or not. An employer is not allowed to use tips to top up wages to the legal minimum.Therefore, unlike in much of North America, the need and culture for tipping is much less.
Equally, British people have a reputation in the USA for not tipping, and indeed many object vociferously to the practice, arguing that the cost should be included in the price of the food. Others however feel it appropriate to tip in various scenarios. As a general rule, it is very rarely expected that one tips in the United Kingdom, unless the service is above standard, but it is polite to forgo change, as a comvenience and politeness.
Polite? yes. Convention? nope.