Other than metallic switches, or possibly light fittings above a bath or similar, where it is possible to touch the fitting while being in contact with something at earth potential (radiator, taps, etc) it is hard to see how it would be a C2, which is basically what BPG4 says (quoted below is for C2 rating):
- Absence of a circuit protective conductor for a lighting circuit supplying items of Class I equipment, or connected to switches having metallic face plates
So I would go with BPG1 and verify there is no current issue with insulation and change the light switch plates to basic plastic ones. But only if it was the only realistic option.
Edited to add: While not directly related, 410.3.9 (iii) has provision for reduced protection for small parts that present a low risk of significant contact or the ability to be gripped, so things like the face plate screws don't really need covering as they are unlikely to allow enough shock current to be dangerous (assume the person is not soaking wet, of course).