But if they ask why I haven't said anything in the past and I say that i haven't been driving they might check the system and see that i was insured so must have been driving, then what, I've owned up to a crime and may get fined thats what I'm worried about.
The database of insurers works more on the vehicle registration than the named policy holder.
If they ask specific questions you have to answer them honestly, if you decide to bluff it and lie, when they check your medical references for a start date they may discover differently.
In short only answer what they ask and say you've not driven since date of diagnosis because your partner drives.
Regarding not telling them because you didn't know you had to, in short a court would take the line that ignorance is no defence in the eyes of the law. If you didn't read that stealing was a criminal offense and stole a pie would you have any defence by saying you didn't know taking someone elses property was stealing?
I'm sure you'll be ok, I am more unsure of the penalty they could pass or prosecution they could bring for not advising them, having not driven and as such not having made an insurance claim in the period.
Of course, if you've insured a vehicle that normally means you could have been the registered keeper?... if so, then they might know, but while DVLA is one body with two sides (driver and vehicle) their interchange of data is generally better with the passport office and insurance companies than their own systems. Again the registered keeper doesn't have to be the legal owner.
It's better to come clean now than continue in ignorance and get caught if you are ever involved in a road traffic incident.