Hi Christine,
Like yourself when we visited Spain this year we did not want to risk a fine.We went through border control and a police check point and the plates were never checked or commented on. They were not quite the correct spec,but near enough.We never saw any other foreign vans with plates on.In hindsight I would not bother again with the plates.
Fixed mine on with number plate self adhesive pads from Halfords.They are still on my van from June and I have traveled around 3000 miles,so the pads have certainly held well.
Nidge