I know, but you often have a better insight than most.
Yes, you have to declare most medical conditions.
You are imagining things that are not real again.
You really do not need to declare most medical conditions to an insurer. There is a defined list of 7 conditions that might need to be reported to DVLA. Sometimes it is only necessary after a doctor instructs you to e.g. most people with well managed diabetes will not need to inform DVLA.
The list of the conditions is at https://www.gov.uk/driving-medical-conditions - you will note the 7 conditions that are listed. I believe the Black's Medical Dictionary lists thousands of other medical conditions that are irrelevant as far as motor insurance is concerned.
Sometimes, as with my sleep apnea, you are told by a doctor to self declare but DVLA will write to the consultant. If the consultant writes back and deems the condition well managed you will get a letter back from DVLA saying they are satisfied your condition is well managed and your driving is unaffected. This check will need to be repeated in 3 years but for those 3 years I do not need to declare this condition to insurers.