Well you are thinking about critical illness insurance, there are so many things to think about and to understand and this is on top of deciding what contract to go for and with which provider.
Well first things first do your research ! Gather as much information as you can about critical illness insurance first, read about the illnesses that are covered and understand what will consitute a pay out and what will not.
There are literally hundreds of plans available, all with different quirks and different parts to them. What you need to understand is that the majority of claims are for heart attack, cancer and stroke and because of recent change in legislation within the UK the ABI have made these compulsary inclusions in all plans.
Critical illness plans have anyhwere between 25 to 40 critical illnesses covered within them and it is important that you understand what is covered in the particular contract you have decided to take. Ask for the policy summary and many providers can give you a provider coverage chart, these are fantastic at showing you exactly what is covered in each individual contract and you can see right in front of you who covers what and the comparisons of both.
An interesting part of the majority of critical illness contracts is childrens cover, this means that if you have a child that is naturally yours or legally adopted they too are covered under your plan. They will not normally recieve a full payment it is normally worked out as an average of the cover normally upto about £20,000. The children are covered for all of the illnesses in the contract and you can have upto a maximum of 2 claims throughout the lifetime of the policy.
In conclusion get your research done and understand your options first.