Speeding is a criminal offence. However, it is not a recordable offence. A convicted offender would not have a criminal record, but DVLA records the outcome. 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 the list.