That's being generous. The essential rule of thumb is 15% of the pre-tax amount, rounded up to the nearest dollar. Hence a proper tip on a $42.40 bar tab ($37.15 + $4.85 tax) is $6.00. More if the service was excellent, or if you were a large and challenging party, or if someone spilled something on the floor, etc.

Old preconceptions die hard. A lot of Americans still think British cuisine begins and ends with bangers and mash, mushy peas, gluey gravy, heavy / fatty roasts, Heinz beans on toast, etc. They have missed the Brit food revolution at both ends of the price-sophistication scale. The same might be said for Brits who delight in dismissing American food as fried, starchy, mass-processed crap because they saw someone say so on television back in the 1980s.