I figure there's no way there'll ever be a team in the UK (or Europe for that matter) unless the relevant government agrees to give the team exemptions from local labor laws. (Can't see how it could be otherwise, especially with the draft, salary cap, etc.)
Sporting occasions will always get special dispensation. Take the recent Olympics in 2012 whereby special deals are made to hold these events regarding taxes. The same is for Wembley stadium when it has been hosting the Champions League final whereby they basically offer UEFA a tax-free game to host the game. Its a win-win, organisations make more money, London and the UK sells its City to the world and therefore entices millions of tourists every year... You could even add the current International series games that already takes place, I'd bet there is a deal already in place with the NFL and our Government in order to have these games.
The schedule seems to be the biggest argument against, but what if the team has two bases, say Miami and London, say it has one on the West coast too. It doesn't have to be like my fellow UK resident above says blocks of 4 games, it could easily be 2 games.
As for traveling distances, take our Footballers that reside in the UK, there are many Brazilians, Argentinians (South Americans as a whole) that make grueling 12hr flights from the UK to play international games 24hrs after stepping of the plane... many fly first class and they sleep overnight on the journeys, Liverpool themselves hire a Lear Jet for their two Brazilians.
What's stopping the NFL purchasing an Airbus A380, decking the thing out with 60 beds/rooms? After all, the bloody thing can hold 853 passengers!
Also every year there is a disparity with regards to teams and mileage, some travel much more miles than other teams, I'm sure there is a table out there.