Not specific to Mexican restaurants or London in particular but I question the authenticity of any exotic (defined as anything not indigenous) cuisine far away from its native home where there is no immigrant community to support it.

Was in France for 6 weeks and noticed that there are Viet, Chinese and Indian restos, some of which serve all 3 of the cuisines (!) and others to boot (Thai, Cambodian and others, even French). Needless to say I wasn't desperate enough to try any. The Mexican angle did get interesting in SE Provence as there was some locality (forget which but maybe Nice) where two local brothers went to Mexico and made a fortune about a century ago, inspiring others to follow in their footsteps. Not sure if that made any lasting connections culinary wise.