wmoore wrote:
I... I then noticed that below his number plate he had a electronic LED scrolling sign that said 'Why the need to rush ?'
Or perhaps that tells you that he is not empathetic. What does it specifically mean too ? It is a very open question and will distract road users (as it did you) whilst they contemplate it and all it's variables. It may also lead some road users to behave deliberately differently too that may not be good.
wmoore wrote:
So am | allowed a sign on the back of my car saying 'Down with Speed cameras'

If I understand you correctly those signs can be bought from places like Maplin, however whether they are legal I very much doubt it as it can be very distracting for a pretty significant time frame to 'read' a sign.
The sticker type 'sign' can be on your car as it is 'static', whether there is a limit to those I am not sure, but it is rare to see any vehicle with more than a few stickers.
If BIB were on the road I am sure he would be getting pulled.
There is the danger that when one starts, it before we know it many will be 'texting' other drivers and that is not good for road safety.
I can understand the desire to promote one's own thoughts to others, but these thought processes are limited by our own 'life boundaries' and everyone's is different.