I think if Roupell Rd were open, I could get to Tulse Hill / West Norwood avoiding the gyratory by going Athlone, Roupell, SCR, Hillside, Palace Rd. By closing Roupell and Upper Tulse HIll they keep all traffic from Brixton to West Norwood / Crystal Palace on Tulse Hill and the South Circular.
If one of those two were open then life in the other roads in the LTN would be unbearable. They are displacing the traffic onto the main road instead of onto the side roads.
I completely understand what they are trying to do, it definitely interrupts my journeys, and I don't know whether it is worth it.