You really have no idea how tenuous employment for people is in those situations, do you? They don't take these jobs because they pay well, they take these jobs because that's all they have.
What they're going to think is "what a fucking asshole". Zero tipping is reserved for the sorts of dickheads that leave "Jesus money" instead of actual cash.
Minimum wage in the UK sounds a lot higher than minimum wage for wait staff in the US which can be as low as $2.13 in some states.
£10 is $13.29, which is a hell of a lot closer to $15 than $2.13.