I too would be very uncomfortable with the idea of introducing a live conductor from the ring circuit into a light switch box. (not least because a 30A protected circuit would be running trough a 10A rated switch) But, isn't it the case where often there are two separate lighting circuits in the same switch box? Take a two way landing light for example, where the supply comes from the 'downstairs' lighting circuit and is present in the upstairs switch box. Also present in that upstairs switch box maybe a a supply from the 'upstairs' lighting circuit which is switching extra landing wall lights for example. Is this against the rules, just bad practice, or perfectly acceptable??