To use a single even over sleeved is against regulations, so is a wire not insulated as with twin and earth, it states "Single-core cables that are Coloured green-and-yellow throughout their length shall only be used as a protective conductor and shall not be over-marked at their terminations" since using multi-core that does not apply. I think it should be against the regulations, but with my BS7671:2008 it's not. Maybe latter additions it has changed it was 514.4.2 Protective conductor if you want to look it up. And yes I would want to use 3 core and earth cables, not 2 core and earth, and even if using 2 core and earth the green/yellow should be over-marked. And it is clearly breaching the regulations not being over-marked at least.