Playing ...Bf5 against the Queen's Gambit is quite possible, and is known as the Baltic defense. GM Igors Rausis has had decent results with it decades before getting involved in a cheating scandal.
In its ideal form, you play 2...Bf5 before committing your c-pawn, keeping ...Nc6 as an option, and possibly defending Pd5 with ...e6
Of course, if White has ...
The London Defensive System is actually a named line in the Réti Opening. And yes, it is very playable. You don't see this very often nowadays, because White will try to stop Black from playing it.
Here is a famous game Réti - Lasker, 1924, in which Lasker went for this line and won convincingly.
Look at the position after 9.Nbd2.
From my analysis:
I tried this about 15 years ago and after 4. cd cd 5. Qb3 I was soon in a world of pain. I've never tried it since.
1. d4 d5 2. c4 c6 3. Nf3 Bf5 4. cxd5 cxd5 5. Qb3
There is no good way to defend the b pawn and white's minor pieces flow effortlessly into the attack. Nc3, Bf4, maybe Ne5 at some stage, either e3 or even e4, followed by Bb5. Nb5 ...
That is the Slav, but the problem for black in many of these lines, and specifically immediately, is that if 3...Bf5, then 4.cd cd 5.Qb3 forces you to sacrifice d5 or b7 since 5...b6 just loses.
1.d4 d5 2.c4 c6 3.Nf3 Bf5 4.cxd5 cxd5 5.Qb3! b6? 6.e4! (6.Nc3) Bxe4 (6...dxe4 7.Ne5+-) 7.Ne5 a6 8.Ba6! Ra6 9.Qb5 Nd7 10.Nd7! Qd7 (10......