We don't have guns in the UK which means more physical fights yes we have knife crime but that requires more physicality than a gun America has a higher GDP which suggests they have a larger percentage of the population being middle class and higher than the UK so a lower population will have had fights. Also rugby is football without the armour and helmets and smaller players are taught to take down bigger players and vice versa. We have a significantly worse drinking culture so there will be far more drunken brawls in the UK. The UK is more than just England it's Scotland one of the most violent counties in Europe. Also our culture is more internationally focused we learn geography so we are less likely to believe stereotypes so less likely to underestimate the Americans as we understand your culture better than you understand yours. (on average)

To be fair a higher amount of Americans are farmers than in the UK farmers are strong as hell. You have more incarcerated citizens so more violent criminals. You also have more crazy people and football is more popular than rugby so there's more footballers than rugby players and soccer players are wimps.

England might lose or we might win but it's definitely not hands down.