Sorry, but I am in disagreement with most posters here. An NFL team in London is not just likely but inevitable.
There are at least three failed or failing NFL teams - the Chargers, the Raiders (whose move to LV will be an all time epic fail) and the Jags. The Raiders are stuck with LV due to the new stadium, they will not be able to walk away from this. But the Chargers are simply not financially viable in LA, whatever Spanos wants to believe. The correct move is back to SD but with him in charge there is no chance of this. The Jags have always been ultra marginal. Something is going to give on one of these two teams.
I don't buy the view that UK NFL fans won't support a London franchise because of existing commitments. Easy to say until it happens - there is plenty of support for an NFL team and people will easily become supporters of two teams. Truth is the NFL has no growth prospects in the US - Canada and Mexico, whilst closer, are not as financially viable as London. Eventually, London fans will get sick of neutral games (especially with poor teams like the Jags constantly being sent) and the interest will wane. The only long term solution is a franchise, the league wants the growth and money talks.
The tax issue will be easily solved - it is a bit of a myth that taxes are way higher in the UK anyway, but teams will just structure pay so that players are not ordinary resident in the UK. The UK Govt will adjust as necessary. Players will do training camp in the US and just live in the UK during the season. The issue is the schedule. I imagine that a London team will have to do a series of homestands and road trips (Say three home games followed by three away games) to cut down the travel. With careful use of bye weeks, it is doable. The World League managed the logistics thirty years ago of having mixed European and US teams. If they use the Tottenham stadium scheduling is not an issue as it has Fieldturf, so they can play soccer Saturday and NFL Sunday with no issues.
The biggest problem I see is the playoffs. If a London team is denied home playoff games because of the scheduling problem, London fans will not accept it. No byes in the playoffs so teams will just have to suck up the travel if a London team gets a home game. It is an 8 hour flight and professional athletes being paid millions of dollars will just be told to suck it up.
As far as FAs go, I think the idea players won't want to play in London is overstated. NFL players don't get to pick and choose. If you get to live in the US all year round except the season, and half of this is spent in the US anyway, is it really that big a deal?