The trouble with the Hornby Black 5 (and the Dapol A4) lamps is they're in fixed positions. As everyone knows, in the UK, lamp positions changed according to the type/class of train being hauled.
The Southern group of railways were even more complex, as they mostly relied on disc headcodes according to the route taken by the train, and you could have anywhere between one and three discs showing on almost any combination of six lamp iron positions. Dapol are trying to address this in their new Bulleid Light Pacific, which has six switchable headcode lamps in front, and the same again on the rear of the tender.