London System Study
For many years, I myself played the London system. Even today, though I have moved on to primarily the Queen's Gambit, occasionally play the London system. I find the opening to be very solid and allow for rapid development. When I first started getting serious about chess, the London system was a good opening for me. It got me away from the boring (yes, I found it boring) e4 e5 stuff. Especially with the introduction of the Sicilian Defense (1e4 ... c5). I did not like this and I found this opening to fit my style of attacking and play quite well. For a few years I used it until I decided that the Queen's gambit was more fit for me as I continued to play. The London system is still in my opening repertoire and is used sometimes. Even when I play the Queen's Gambit, 3. Bf4 stands out to me and I sometimes prefer to play that. So, if you are looking for a solid opening, the London System is a perfect one to learn! It is also worth knowing how to defend and play proper incase if someone plays it on you... Then, you'll know how to exploit it when the mess up a line. Check it ous and see how you like it.
London with 2... c5
Yes, for most beginning London System users, this move is scary. It is actually the most common approach and possibly the best! c5 challenges the center pawn (which white did not do and instead focused on development, which again is by no means a bad thing!).
The Tarrasch Approach
Challenging the dark sqared bishop is common for black to do. In some cases leaving the bishop on b4 is actually a decent move and can compensate later on. Moving the bishop away can waste time but is an acceptable alternative.
The common London System
This is the common setup in the london system. Notice the bishop and queen's aim on the pawn at h7. After Ne5 black can not take, otherwise the pawn takes and the knight leaves to lose a pawn. Either way, this is a solid position for white to play.
The Slav approach
The solid center formation for black will allow for rapid development and a flexible queen bishop.
Symmetrical London
If your opponent replies with Bf5 on move 2 or 3, then playing pawn to c4 right away will do good. This is because the bishop is no longer on the queenside to defend so you must go for it and create weaknesses. Bb3 should also be played soon because of the weakness created by the absent bishop. Even if Qb6 in reply and later Qxb3 the pawn structure leaves white with excellent queenside chances.
Grunfeld Formation
This is a king fianchetto line. It is common against the queens gambit, and, revised a litte, can work here too.
The Pseudo Old Indian
This is interesting and a lot of times is caused because of white's second move. Black may want to transpose into a kings Indian. Either way, this scares London users because it is way different than any of the pther setups. The line is super flexible for both sides and the setup can very based on the move order. Also, the queen moving around so much is a waste of time. White can sometimes give up the b2 pawn for a time gain and should have plenty of compensation.
The dutch reply
Some players play the Dutch and won't give it up just because of the London system. Rushing the kingside pawn forward has a certain charm. Black has to take care the first 20 moves or so, but don't expect to refute the Dutch with pure brutality.