If you are a member of a trade union, you may have legal advice (and possibly even representation) as part of your membership.
It's my understanding that ambulance drivers (not meant to be inflammatory, but meaning anyone who drives ambulances) have to take advanced driver training courses, so would you not be expected to be more aware of the speed limits that are in force, and to have more control of your vehicle? I do understand that humans are human and sometimes we creep over it, especially with road designs that imitate much faster roads but end up being 20mph.... (Looking at you Wales!).
As for your memory - again is this not something that may affect your work, if it is affecting your driving to this extent?
These are all questions that a judge (or your employer) may ask you, so worth exploring them now when not under so much pressure to do so.