Here's the definition of a "gammon" from EngLangBlog, a blog for A Level English Language students and teachers:
"So what does this new type of 'gammon' mean? It first appeared on Twitter in 2016 in association with David Cameron, whose face was on occasion described as resembling a boiled ham (and after that story about Cameron and a pig's head had been circulated, it took on a slightly more disturbing tinge). But it was in 2017 that it really took off when it was used to describe a post-Brexit vote phenomenon that many had observed but few had been able to nail so accurately: namely, a certain type of man whose anger about the modern world had made them turn the colour of ham or what Urban Dictionary's (now mysteriously removed) top definition describes as "a particular type of Brexit-voting, middle-aged white male, whose meat-faced complexion suggests they are perilously close to a stroke".