although not explicitly stated in the rule governing number-plate fitment i believe there are a number of rules that come together to make putting it in the screen illegal
firstly it has to lay on an angle which combined with the screen makes it more difficult to read the letters it also has to be 'Fixed' to the car which means sticking or bolting it to the dash.
also anything that obscures your vision out of the front screen while driving above what is fitted as standard, has other rules which can come back to bite,
you could try arguing with PC plod but i strongly suspect you wouldn't win if he took umbridge to your screen mounted plate. which you could face losing if he reports back to the DVLA that you are displaying incorrectly.
its basically easier to screw it to the front of the car. it also looks better and doen't make other drivers think its fallen off
if you want to do something different try a smaller bike sized plate or mounting it alfa Style but you still might find you have problems with the police thinking you are trying to hide something/it
http://www.legislation.gov.uk/uksi/2001/561/pdfs/uksi_20010561_en.pdf
What looks better on the dash or drilled into the bumper?? Ive got a black colour coded bumpers, what you think? Thanks guys! 
