I know it may not make them much smaller, but you can trim the ends off of the plates.

As long as you have the same amount of room as you have at the top and bottom of the plate and replicate this on the ends, you will be fine.

The biggest problem with smaller plates is the spacing.

And, annoying though it is to get pulled (been there too ) it is a damned sight better than waking up to find your pride and joy missing, ringing the old bill to tell them it's gone and then their ANPR cameras not being able to read your number plate and stop the thieving chavvy gypo bastards?