What you are looking for is referred to as "clear bra" here in the states, and popular brands are 3M and Versa-Shield. I can only dis-recommend Versa-Shield, as I have it all over the front and bottom of my sparkly black Dodge Magnum SRT8, and the Versa-Shield product has begun to dull and yellow. I have seen the 3M product on other cars of the same color, and it is much better.
On cars with non-metallic and/or lighter colors (white, gulf blue), the clear bra totally disappears, and the only way you can find it is to look for the fine line of dirt that collects at the edge! On dark colors or metallics, however, it can be tough to maintain invisibility....