See I think this point confuses many Americans. Over here, you have teams that draw from literally 500 mile / 1,000 miles radiuses (radii)? because we are so large. So I'd bet that the NFL is thinking (if they do something crazy like move the Jags to London), that essentially they'd be drawing not only american football fans from London, which obviously they'd need to be their base, but they'd be thinking they're the team of the UK, and it's nothing for American sports fans to plan weekends and longer trips around seeing their team play on the road. For us, it's part of our culture. For most of us, it's unheard of that you would wake up early in the morning, take a train or coach down to London or the south coast for an afternoon game, and then not stay the night there. Travel over long distances is just part of what we do on the regular.
I bet they think if the UK has 1 million real football fans and another 2-4 million that are kind of into it, then that's plenty to support a team
I think you have to factor all sorts in.
I think it's extremely obvious that Football is america's most popular sport, by some distance. Compare it to the UK where it's clearly Football, but our kind, and you will see the comparisons. Fans go up and down the country watching the games because it's what they do, same with NFL, but the big difference is volume.
The NFL season is at max 20 games, with a schedule announced 5 and a half months before the season starts, including TV Fixtures. Fans can plan ahead, in football, we can't really as for example, we have a game in 6 weeks vs Norwich which may still change. Then factor in that we 38 games, plus cup to factor in.
In America it's ridiculously cheap for fuel, the roads are grand and it's easy to get about. But it's different. It's just different, you'll know yourself how the sports are different. Everton planned a move 5 miles away to Kirkby, and there were protests, as the club wouldn't leave the City. Look at American sports, Jets and Giants play in NJ, The Niners relocated 45 miles away to a different city. Imagine Everton moving to Manchester?
British fans outside of London will not be flocking to Spurs every other week (in reality, it will be blocks so probably consecutively), there won't be an association. Like you assumable are a Steelers fan, would you change allegiance from Everton to a new PL team if they moved to NY? No chance. It's a nightmare really because when it fails, we lose all games from the UK, which is a pain.