While I don't recognise it myself, it may help others if you can give us a sense of scale. How large is it?
Is that rust or just muck? There appears to be a hint of green on the bottom of the 6 which would suggest brass.
The number doesn't match with any BR locomotives. The range 80000-89999 was used for BR standard design tank engines, but no numbers were allocated above 84029, according to this list. I can't think that any pre-nationalisation companies would have numbered their engines that high.