With just a little effort, we've had pretty good results doing the same project here in the U.S. The plnaters were fairly large... perhaps 20 inches tall, and 18 inches each side.
After thoroughly washing them with a power washer, I used a medium grit metal ("wet") sandpaper, followed by a medium steel wool to lightly etch the surface. After drying I used this product:
http://www.misterart....aint-for-plastic.html ... although I live in the U.S. a similar product should be available in the U.K. Did it last summer and they still look pretty good after being stored outside in our western U.S. winter. Tip: apply the spray paint on the inside down to about where the soil level will be for a nice, complete finish...