The medical questions about cancer will usually be "have you had treatment for cancer in the last five years?" . For you, until early April 2028 the answer to that is, "Yes". If you don't like the look of the premium then most insurance companies will let you exclude an existing condition, i.e. prostate cancer. Prostate cancer is unlikely to be a problem on your holiday, and I assume you have no complications from the operation because they will be excluded too.
I was treated in September 2018, I have excluded prostate cover since then. I assume after September 2023 it will become irrelevant for the insurance companies, and I won't have to declare it, though in reality prostate cancer can return at anytime.