I think this bird is a juvenile Besra. It seems to be relatively small and slim, which does not favour Crested Goshawk. I also think the bird is too heavily marked below for a juvenile Crested Gos, which usually seem rather pale and spotted.
I find structure to be the best way to separate Besra and Japanese. Japanese usually appear longer-winged and shorter-tailed, compared to Besra which has rounded wings. The wing on this bird appears to be fairly rounded - the longest primary appears to be P5 (i.e. the 5th feather, counted from the outer wing). This favours Besra over Japanese, which should have the wing point at the 4th primary, notably longer than the other feathers. I am not sure about plumage differences between the two, but I think that the plumage fits OK for juvenile Besra. Also, of course, date does not favour Japanese which has never previously been recorded in summer.
In terms of location, I think the suggestion that Japanese is more likely on offshore islands is misleading. All the HK accipiters have turned up on Po Toi and other offshore islands, and Besra seems to be at least partly migratory through HK (with peak numbers during passage). I would think that Tung Ping Chau provides suitable breeding habitat for both Besra and Created Goshawk, and either should be possible there.
