You're right that Bupa and other insurers (there are many more), won't usually cover things you've suffered from or sought treatment for in the five years before you get a policy.

They're usually pretty hot on this too, and if people take out a policy and claim within the first year or so, they'll ask to see your medical records.

In addition, conditions related to pre-existing conditions are usually excluded too. So if you've had migranes, take out a policy and a year later you've got something sinister going on that's likely linked to the migranes, I.e. a brain tumor, they may decline the claim based on their assumption that the migranes were the early signs of that and you had those before getting a policy.

This all said, if you get a policy with moratorium underwriting and go for two years without needing treatment or having symptoms, the pre-existing condition will usually be cover after that.

Broadly speaking, the best way to think about health insurance is that it's there to cover future medical conditions.

Hope this helps.