You have to answer truthfully any question you are asked when applying for insurance. You do not need to volunteer information not asked for!
The Government and the ABI have a policy framework (‘Concordat’) for cooperation that includes a voluntary Moratorium on insurers’ use of predictive genetic test results (NOT diagnostic tests) until 1 November 2019, (to be reviewed in 2016). So for most tests, companies cannot force test before providing cover, customers do not need to disclose result while insurance in place, and do not need to disclose results of blood relatives.
Only one exception currently, for cover above £500 000 and Huntingdons. Certainly no need for time limited policies eg travel, private medical care; really just for life insurance, critical illness and income protection. Recognition that increased risks of a small minority can be mitigated by larger population of policies  Evidence from US is that significant proportion conceal their diagnosis.
Asymmetry of information—when the customer knows more than the insurer—is the industry’s nightmare. Testing positive for ApoE4, a mutation of a gene related to increased risk of Alzheimer’s, would be a good reason to get life insurance before symptoms develop.
See also ethics.