Either in the 17th or 18th centuries think of a remotely plausible way in which the coast of Chiapas (and hinterland if you'd like, but it doesn't need to be the entire state) could conceivably be colonized by the British. It doesn't have to be peaceful, it could be obtained by force against whomever controls it.
You can then feel free to speculate on what sort of colonial policy the British might enforce (personally I'd wonder how multicultural it could end up as, e.g. like Belize or Guyana) and what effects it might have on neighboring countries, but the main thing is the AHC.
You can then feel free to speculate on what sort of colonial policy the British might enforce (personally I'd wonder how multicultural it could end up as, e.g. like Belize or Guyana) and what effects it might have on neighboring countries, but the main thing is the AHC.