Speeding is a criminal offence. A contested speeding matter would be tried in a criminal court.
However, it is not a recordable offence. A convicted speeding offender would not have a criminal record; the DVLA records the matter. The conviction is not recorded on the Police National Computer as a criminal conviction.
Imprisonable offences are recordable offences but speeding is not an imprisonable offence. There is a list of non-imprisonable offences that are recordable offences; speeding is not on that list.