I've been looking casually for evidence of Mensur style fencing or dueling blood sports in areas other than Northern/Eastern Europe. Is there an equivalent in Spain, Italy, France, or Britain?
No, not really. Academic Fencing is fairly unique, and you don't find similar traditions elsewhere. German students who studied in England during the period did, apparently, comment on the similarities with the English students' propensity for boxing, but superficially at best, especially as, to my knowledge, there wasn't the same sort of centrality to boxing as a part of student life, or the underlying fraternity system to promote it - not all German students joined fraternities, let alone ones which dueled and made that a defining part of them, but roughly 1/4 of German university students were members of one which did.
So that's it really. Not much more that can be said. I'm sure you can find other activities that, like boxing, have superficial resemblances, but I don't know of any serious Academic Fencing in the countries you are asking about, or another blood sport carried out in the same systematic manner.