Wembley to now host an additional 4 games, as Brussels couldn't guarantee their stadium being ready in time, and UEFA decided not to award the games to a city who lost out in the initial bidding process...
http://www.bbc.com/sport/football/42270759
I honestly have no idea how this is going to work out, what with visas obviously being potentially required at incredibly short notice for games in the last 16 and beyond.