1.d4 Nf6 2.Bf4 c5 is my preferred line. Most London System players refrain from 3.d5 since it breaks with their usual plans and opt for 3.e3 or 3.c3. Here 3...Qb6 are both strong for black, though be aware, if playing a lower rates opponent that there is an easy draw after 3.e3 Qb6 4.Nc3 (or 4.Na3) Qxb2 5.Nb5 Nd5 6.Rb1 Qxa2 7.Ra1 Qb2 8.Rb1 etc.
Good reply for black to the London System
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Shankland's line 1. d4 Nf6 2. Nf3 g6 3. Bf4 Bg7 4. e3 d6 5. Be2 (or something else) 0-0 6. h3 Nfd7!? 7. 0-0 e5!? is worth considering for KID players.
A possible continuation is 8. Bh2 Nc6 9. c4 f5 10. Nc3 e4 11. Nd2 Nf6 with a highly dynamic position where black can play for a win.
The King's Indian defense is probably the best response to the London system
1.d4 Nf6 2.Bf4 g6 3.Nc3!? d5 (3...d6 or 3...Bg7 lead to a rather unusual Pirc after 4.e4, but still this is not plain sailing for Black) 4.e3 Bg7 5.h4! is a "Barry attack improved" (the knight is at g1, not f3). This might be equal, like the genuine Barry attack, but Black has to be careful.
I agree, IM Pfren; But we must first understand the mind of a amateur and they are fearful bunch and any aggressive move is to be avoided. The aggressive 3.Nc3 is not the spirit of the meek soul of London system; But if I meet a player with a adventurous soul 3.Nc3 d5! 4.e3 c5! ( the natural 4...Bg7 invites trouble with 5.h4!) and black has nothing to worry about.