Safespeed.org.uk has been following the progress of a case in which a man had a speeding prosecution thrown out because he didn't sign the form identifying the driver.
The legal loophole seems to be that you are obliged to identify the driver when presented with a form, but you aren't required to sign the form. Bizarrely though, the form is inadmissable without a signature.
Check out the full story here: www.safespeed.org.uk/unsigned.html