In the US I think the wages for waiting staff are genuinely so low that tips really are how they survive so it's expected. Not leaving one then, would be making a point that the service was exceptionally poor.
In the UK however, the wages are exactly the same as many other jobs that aren't even in customer service and so "getting tips" is impossible... the pay is the pay. Personally I do tip if the service has been noticeably and genuinely (i.e. not cheesily) ABOVE average, but not otherwise
