The PET702 will be a region 5 machine, whereas the UK DVDs are region 2.
DVD players from a particular region are designed to play only DVDs from the same region, due to copyright and distribution rights in each country or region.
However the PET702 can have the region protection removed by making it multi-region:
Switch on the PET702 and Open the tray.
Press SETUP button on the remote control, which will bring up the menu.
Highlight "PREFERENCES" and press OK.
Press , , , , , , [LEFT], [DOWN]
The screen will show: "Region Code: 2"
Press [1-6] to manually set the region OR  for multi-region.
Press [SETUP] to exit
You will now be able to play DVDs from anywhere in the world.
This is particularly useful, because region 1 DVDs from the US often come out when the movies are still in the cinemas over here. Region 1 DVDs can be bought perfectly legally by mail order or from the internet.