Alex Banzea Teaches The London System | 1000-1100 Part 1 

On move 29? En passant is not legal there. A good way to remember en passant is that if you can push your own pawn one square and the opponent can't capture the pawn normally, then en passant does not apply. Or just remember that the capturing pawn must be 4 squares away from promotion.

En passant not legal on move 29.

I suggest you get Alex's Chessable course. But in short, Bf4 first allows for immediate c5 by Black which is more problematic for White.

to stop e4