I would say the BPG#4 guidance seems quite good. but it always comes down to a judgment call on the risks involved.my personal opinion is lighting without rcd protection should be C2, but under current guidance and regs it is C3.
So I would say C3 for no RCD on indoor lights generally (assuming sound CPC, etc) as you rarely handle the related equipment (less so today with LED lamps lasting an order of magnitude longer than filament lamps). Even metal switch plates, the obvious case, are not something you can grasp under shock conditions and it still needs a double fault to present such a risk (CPC open then fault to metalwork).
But you can easily find examples such as outdoor lights and light switches where the presence of water increases the risk of shock, or low height 'artistic' lighting metalwork that is easily grasped close to other conductive objects, etc, that change the risk greatly and then C2 is reasonable.