this is certainly a cross and the cream legbar thought is on the right path. reddish breast of this and i believe also of the cream legbar is from the brown leghorn, but the silver grey dorking also has this red breast. This red does not get over rode by the silver gene. But a silver gene is involved too. It would be hard to say what is involved otherwise, could be white rock. Likely something white by the paleness of this bird. Or it could even be only a 1/4 brown leghorn, very pale and not much red breast.
I had a hen that looked very much like this whose father was a white wyndotte and mother was brown leghorn. mine was a little darker but the very same color pattern.