I'd say in general the gulf is huge, Mexicans are the biggest fight fans on the planet imo no contest
Boxing is a major sport in Mexico. Probably the 2nd or 3rd most watched sport. Mexico respects and takes care of their top champions. Chiquita Gonzalez, Julio Cesar Chavez, Marco Antonio Barrera, Erik Morales, Jorge Arce, Juan Manuel Marquez, Finito Lopez, and I'm sure more Mexican ATGs are all doing fine living good and are celebrities in MX today.
UK has loads of sports more popular than boxing, the casual fanbase of football fans/other sports fans that only watch Joshua's fights confuses people into thinking Boxing is a major sport in the UK when it's really not.
For big fights the UK beats everyone we will travel anywhere for Hatton , Calzaghe etc. But we are not consistent , those Mexicans will pack out petrol station forecourts between two novices. Boxing is bigger in Mexico without question.
Mexico for the reasons posted above. Not that there's not lots of passionate uk boxing fans, it's just it's one sport amongst many and it's mostly hidden away behind tv paywalls.