Having been a life insurance agent for 30 years, here in Australia, and now retired for 5 years, I also know a bit about Income Protection Insurance. Please note
that my comments relate to Australia and my not be applicable to UK policies.
Agree with all that Nigel has written and add the following:-
Policies can be either "annual renewable" or "guaranteed renewable". Annual renewable is like your home or car insurance in that the insurer can refuse renewal or change the terms of the policy because of claims history or change of occupation or pastimes. Guaranteed renewable is that irrespective of claims history or change of occupation or pastimes, as long as you keep paying the premiums, you are covered.
Waiting period and benefit period also affects the premium, ie the longer you have to wait from first receiving medical care following an injury or sickeness to when you start receiving a benefit payment reduces the premium. Conversely, the longer the benefit period, be it 2 years, 5 years, to age 60 or 65, will increase the premium. Some policies also have a specified injuries/sickness benefit where you are paid a minimum benefit irrespective of the time you are off work. For example, 5 years ago when I had a prostatectomy due to cancer, I received a 6 month benefit (with no waiting period) even though I was only off for 1 month.
Some policies may also have a "new in business" clause which may restrict your initial benefit.
Some policies also have a "stepped premium" which means that each year, your premium increases with age.
You may also be offered a "level premium" to a specific age which means, other than for increases in sums insured, the premium will remain level. This is dearer in the short term but cheaper over the long term life of a policy.
You may also be able to obtain a "guaranteed sum insured" policy whereby you give proof on income upfront and, then at time of claim, no income proof is required. This is good for self employed professionals whose income may fluctuate due to various outside market influences such as a downturn in the building industry could affect architects. At time of claim, when you are infirm, would you want to be getting your financial records together or prefer it to be done? An employee would not require this as a pay slip etc would suffice which family would be able to attend to for you.
Beware of exclusions! Some policies may have pastime exclusions such as motor racing, skydiving, ocean yacht racing, scuba diving, private flying etc. In a "guaranteed renewable" policy, should you commence a risky pastime after taking out a policy, you should be covered for that pastime.
And remember, full disclosure is required at time application, be it health or pastimes, as one does not want a claim rejected because of non-disclosure.
Hope this sheds some light on the "mysteries of Income Protection Insurance".