So let's look at population size. Birmingham, England has a population of 2.7 million people and fills, as you say, 2 stadiums of 30K seats. You'd think it'd be tough for much smaller 116K population size Tuscaloosa, AL to fill their 100K seat stadium (Bryant Denny Stadium for Alabama football). Obviously, most of the Bama fans going to the game come from outside of Tuscaloosa.
And it's not like the U.S. doesn't have plenty of saturation of American football with the NFL (32 teams) and other professional leagues like the UFL (merged with XFL and has 8 teams), GDFL (29 teams), and RPFL (12 teams). Combine that with 136 FBS college teams. To me that seems like tons of saturation of both professional and high college level teams (209 teams). Not to mention saturation from other sports like basketball, baseball, and hockey to spread out the fandom. As well as saturation of college fandom for lower level colleges (I attend my much smaller alma matter's games more often than I attend Bama's games.)
Yet even with all of that competition against big college football games, our one country can fill eight 100K seat stadiums on college football Saturdays.
One of the things though now, if you go to an FC Barcelona or Real Madrid game, most of the people there will be tourists, who are willing to pay for the $200 tickets. They are pricing out the locals.
That’s the 2 big guys. If you’re going by population size England wins. Most English cities have total stadium capacity of about 25% of the population. And 23 home games a year. Most American cities even if ALL the sports played home games at the same time you’re looking at 2 maybe 3% of the populations.