If there are no cameras, no police around and you don't crash into anything, nothing happens! ;-)
If you're caught by a camera (or seen by the police) and received a fixed penalty, it's the same as for a basic speeding offence. i.e. �60 plus 3 points.
If the matter is dealt with by a court (which would only be likely if the offender appealed against a fixed penalty or if an accident occurred because of the failure to stop), more severe penalties could apply. (However, in the case of an accident, it's more likely that the matter would be dealt with as 'driving without due care or attention' or 'dangerous driving', rather than as 'failing to stop').
Chris