I don't believe nobody has taken DNA to compare with the UK autopsy results. It could be taken from the parents/siblings of the Burmese men. Could a private investigator do this?
Not sure that there would've been any usable DNA left to take...although to be honest I have no idea how these things work, but I wouldn't be surprised if the body was somehow cleaned before it left Thailand. I would think the only way to get a DNA comparison, if there is any, would be as you said...a private investigator..(MOO).