I am not aware of any operational differences in the hdmi spec between USA and UK.
Have you tried connecting the DVD into the same port that the Humax works through? If it is then visible, that would suggest that perhaps the individual (extra) hdmi ports need to be activated in the TV settings.
Humax support informed me that the DVD player would not work connected to the Humax Aura Freeview 2TB box, which is correct. I did eventually get the Pioneer DVD player connected to the USA TV via another HDMI port and it worked fine and has done so since.