This is from the ABI website FAQ's about changes to health.
Do I need to inform my ins
urer about any change to my health?
It is important that you tell your insurer about your health history. If you
disclose something that does not increase your risk of making a claim, your
premium will not be affected. But if you had a condition before you
r trip
and did not tell the insurance company, your insurer may not pay for
medical treatment or the costs to get you home for any claim related to
that condition. To make sure this does not happen, tell the insurance company.
If you do not disclose the change to your health you risk being uninsured.