FWIW I've never seen ketchup in a London Mexican restaurant. Most make the bulk of their trade on Mission style burritos and similar. It's Mexican viewed through a West Coast American filter.
As an example of the sort of corn tortilla that I try to find, here is the place I order from in London when I do not have time to pull out the press and make my own: http://www.coolchile.co.uk/mexican-shop/corn-tortillas