Any of the little shops you see with all the theatre posters around town are all legit and won't charge you much more than face value for anything. You can line up for Day Seats in the morning for all shows at The National. They have some tickets held aside every day. It was last row mezz for Follies when I did it early in the run and I got there an hour before the box office opened. I definitely recommend doing that! You can most likely buy one for each show on a day that has a matinee of one and evening of the other as it's two tickets per person. Good luck!
If you are not already doing so, check the National site regularly. London seems to get a lot of returns and availability changes regularly, particularly for Follies.