Looking on the screwfix site they have 'open' and 'closed' 20mm PVC
grommets - the images look identical, so what's the difference and is
there an advantage in using one type over the other?

The closed ones are "filled in". You need to puncture a hole for the cable
to go through if there isn't one already. This means that the hole is
exactly the right size for the cable, though.

Christian.