I think this bird is much too dark on the back for a Grey-backed Thrush which are usually fairly pale grey. The colour on this bird is actually much more similar to Japanese Thrush.
Also, I think the bill is too bright and the orange on the flanks is too dark and extensive. The spots on the coverts of first winter male grey-backed are usually also paler (whitish), and there is less contrast between the tail and rump.
I have no experience of black-breasted but I think that it is more likely to be that species (it seems to fit better with the images on the OBC website). That would also be more likely on range.
