The Slav and the London system are quite a popular pair of openings - frequently complemented by the Caro Kann against 1.e4. And if my first project had been on the Slav, quite likely the London would have appeared a natural continuation. However, with the London coming first, I didn't really consider following up with the Slav. This isn't quite as silly as it may sound:
While I enjoy playing a reversed opening with White (see my posts on 1.a3) - looking for a good way to use that extra tempo, I find it quite uncomfortable to go the opposite direction - taking an opening which I play with White and play it a tempo down. There certainly will be related lines but a tempo down they almost always are slightly worse.
In the case of the Slav compared to the London, this most clearly is illustrated with this position (1.d4 d5 2.c4 c6 3.Nf3 Nf6 4.Nc3):
a) 4...Bf5?! followed by ...e6 and a typical London position would be Black's preferred move if it was playable. However, very few strong players use this move regularly with Black - probably with good reason as White scores around 80% with 5.cxd5! cxd5 6.Qb3 (a database search hints that there may be more to this than is generally known so I may return to this subject).b) 4...dxc4, which gives up Black's central foothold but wins some time for development, is Black's traditional main continuation. After 5.a4 Bf5 the position has some resemblance of a reversed London. However, the pawn exchange is generally considered undesirable and for London players mainly interesting as an example of how to handle the position if something goes wrong.
c) The Semi-Slav 4...e6, which locks in the light-squared bishop, can lead to very interesting positions if White plays ambitiously but isn't very similar to a London set-up.
d) 4...a6!? - the Chebanenko Slav - is a more recent try for Black. Now when Qb3 can be met by ...b5 or even the exotic ...Ra7, ...Bf5 becomes an option for Black in certain lines. This is a very interesting idea but seen in a London perspective not very attractive. While a3 frequently can be useful in London, it has quite low priority. Consequently Black is not just his normal tempo down - in many respects he is closer to two tempi behind a normal London line.
e) 4...Qb6!? has been played by many strong players, most recently by Kamsky. Black prepares ...Bf5 by defusing the Qb3 option but it must be admitted that this system is more solid than exciting and it seems that White has several paths to a small edge.
f) Black has other options like 4...Ne4, 4...Bg4 and 4...Nbd7 but none of them are very London-like.
