I was in the same boat.
The Good For Age entry is limited to UK residents.
This is a drag as Boston does it the American way and allows anybody in without discriminating based on country of origin.
4 or 5 years ago, I emailed Marathon Tours and was informed that London was their most popular destination and only after paying to be part of the 7 continents club I would have a chance. You're not automatically registered for London if you're part of the 7 continents club.
I decided to give up and forget about it as it sounded like too much money with no guarantee of making it to the London starting line.
Look into it before signing up for London though.
I have read (not certain where) that regardless of your past marathon times, the entrants from the Marathon Tours and other tour groups start in the back.
I would be miserable starting at the back so if this is true then I don't want to pursue trying to get into London.
I have for the past 4 years put my name in for the overseas ballot but have never had success.
I would try it that way if you have your heart set on getting to London.
I believe the2019 ballot opens on the London Marathon web site for about 1 week shortly after the 2018 London Marathon is over.
The ballot might only be open a couple days, I'm not certain.
The chances for success are very small but it costs nothing to enter your name. You give them your email and they inform you in October if you are successful or not.
I have been unsuccessful the past 4 years but, as the saying goes, you can't win if you don't play.