The London System is very much a Slav Defense with reversed colours so it cannot be bad as the Slav Defense is considered a good defence as Black.

Theoretically, White should be looking to gain more space as the first player but in practice it can take the opponent out of their comfort zone. They are more likely to have a Benoni, King’s Indian Defense or Nimzo Indian Defense etc prepared against the more ambitious Queen’s Gambit type of moves. As the London System is more solid it is therefore harder to have something immediately exciting prepared against it. This solidity in the centre can make it easier for White to launch a Kingside attack against a Black castled King.

It is not all great news for White because Black can be flexible in Castling just like White can be which would take the sting out of White’s Kingside attacks and indeed make them weaken their own position. I strongly recommend Black plays pawn to c5 early on like a Queen’s Gambit with reversed colours. That would be a very principled choice possibly targeting the undefended b2 pawn or possibly with the idea of a later minority attack, again like in the Queen’s Gambit.