Holiday Insurance
Please do not be fooled by sites saying you are protected.
Have a look here for help and advice.
https://www.moneysupermarket.com/travel-insurance/holiday-protection-insurance/
https://www.moneysupermarket.com/travel-insurance/enquiry/
As your circumstances are very different from each other, you need to tell the insurer everything.
Your medical condition included what treatment you are having.
Once you take out the policy you are covered from cancellations etc.