So see gp for cough stomach flu etc must declare too?
Wah... your question a bit the challenging.
As grey as the insurance industry is, sometimes buying an insurance plan which acts as "transferring our financial risk" to an insurer, we are taking a certain level of the risk during the application process too. A lot of time is due to faith, such as "I believe I have done my declaration truthfully, and I believe the insurer will not chut pattern to reject my claims 10/20/40 years in the future."
For your question, if you want to be as "by right" as possible and play safe, you are still encouraged to declare, for simple reasons like "Just in case". Sometimes people might laugh at you and find it silly, but it doesn't take a huge effort for a simple declaration. But ofcos, if you last visited a GP for cough 3 years ago, that's probably a too minor issue and it's too long ago to make any sense declaring.
Some insurer would be more specific in their declaration section asking "Have you seek any form of treatment or visited a GP for the last 12 months.", if you visited one due to headache, flu etc, just declare.
But sometimes, things can get more vague when the question phrases in a such a way, "Any other illness/disorder/abnormalitis which are not mentioned above?".