If you have a health complaint that may require any form of treatment abroad, then you will most likely find that your insurance will cost more.  Many people with cancer, for example, will find that their premiums will be excessive. 

Honesty is definitely the best policy with regards to ensuring you are insured for your underlying health condition while abroad.  If you fail to disclose information about your medical history, and then suffer a recurrence of your health complaint while abroad, it is most likely that your travel insurance company will not pay out against your claim.

You may consider that you would be better not to be insured at all, rather than go to all the hassle of disclosing your medical history.  However, remember that if you are uninsured aboard that any treatment you may require will have to be paid for using your own money.  Medical bills abroad can quickly run into tens of thousands of pounds. 

If you are visiting many of the European countries, then you can get medical treatment with an EHIC at a reduced rate.  Even if you have to pay for treatment, at least it will be a lower rate. 

Remember that you will not be the only one that has had to consider appropriate health insurance for travelling abroad.  There are many organisations that support sufferers of major conditions, like cancer, who will be able to provide advice on how to holiday abroad, and they will be happy to advise you on suitable insurers.