I have plenty of experience of a dim charge light which glows brighter under load. In 1976 my Dolomite did this and alternators were £25. A lot then, so a piece of black tape over the light fixed the problem until I could afford a new alternator.
Those Lucas ACR units do vary slightly but I was told sometime later at an auto electrical shop, that the charging circuit is distinct from the warning light circuit. Both are diode controlled, and I recall he said that some charge circuits had three and the warning lamp circuit two. So it was quite possible to have charge and also have a glowing light which glows brighter under load when the diode has failed.
I have no idea if this is still the case. Maybe now a circuit board which replicates the old set up, but it's the "diode" in the warning lamp circuit, almost certainly.
Oh and I've just (partially) remembered that those B's with a seat belt warning light have a diode in the circuit, and if that fails, something weird happens, but I can't remember what.
