Discretionary charge on bills: 12.5% in London, 10% elsewhere per its tipping page
10–12.5%Wahaca's own tipping page states a discretionary service charge is added to bills, at 12.5% in London and 10% outside London. Its current main menu and group menus give 12.5% without mentioning a lower rate, so the rate outside London is not consistent across its own pages.
Source: Wahaca's own published policy. Service charges in the UK are discretionary: you can always ask for one to be removed.