Guess we need to be careful when generalizing colors. Australian carriers seem to have a different shade of rust on their engines when compared with those in North America. Likely they won't match your rust variation in the UK either.
A friend of mine here in the USA will be taking possession of a 1943 dated T-16 with a 46XX serial number later this summer. It never went overseas and so never got any British modifications applied to it. The engine is dead but original to the vehicle so I will post what we find on it in a month or two. I haven't seen it in person yet but the vehicle is still in all original paint so likely the engine is also. It won't tell us anything conclusive but will at least be another thing we can use to see if there is a trend for the T-16s.