To ensure that you receive the best rate possible it is useful to understand how these premiums are calculated by insurers. Firstly insurers will place people into four main categories:
Someone who has a semi-serious illness such as diabetes or heart disease can be insured but will pay a higher premium. People with a chronic illness will be placed in the substandard category. Someone with a terminal illness will be rendered uninsurable.
People with high risk jobs or hobbies will be considered substandard as well.
The premiums that you are charged will correlate with the category that you are placed in. Since the categorizing is not an exact science, one company may place you in a different category than another, thus drastically changing the prices of your premiums.
Once you are approved for coverage from a company, they cannot deny you coverage for any reason unless you cease payment.