Why is expanding the sport an issue? If done correctly, then this could be a great thing for the NFL. Notice I said that it needs to be done correctly, though. If the NFL is willing to actually put effort into the new league franchise, then it could be great for the sport. If not, it could be a disaster.
The NFL has a special place in the USA. Having a London team is the worst idea ever. Players won't like it, fans won't like it, even coaches won't like it. Each team will have to travel a LONG way to even just fly to London. Plus, the country loves soccer way more!
The game should stay on the North American continent except for exhibition matches or the occasional novelty game abroad. Even though there is only one game a week, it does not make sense to have a team headquartered in London with teams traveling constantly back and forth. It is inefficient and may not even be profitable since the English like soccer more.
NFL Europe failed. Granted it was a developmental league mainly and an inferior product, but nobody playing in the league wants this. Glorious Leader Goodell is possessed trying to make it happen and with the sway he has might be successful, but it's logistically a horrendous idea and one I hope he fails at.
I do not think that the NFL should attempt to expand and put a team in London. It would be very inconvenient for that team and its opponents when traveling to play games. They tested the market by having games over there but you cannot be certain. Fans showing up to see a game once a year is different than having a team in a city.