Hi
I have some experience of Income protection insurance both professionally (I used to sell it) and personally (I made a claim on it!).
Income Protection insurance (or Permanent Health Insurance as it used to be called) will pay a fixed or increasing sum of money (usually monthly) to a fixed age (usually between 50 & 65). It will typically have a minimum deferred period (the minimum amount of time you have to be off work ill) of at least 4 weeks but could be 8, 13, 26 even 52 weeks. The amount of monthly benefit that you can insure will depend upon your income or in the case of a self employed person the net profit shown in the annual accounts or declared to HMRC. Depending on the company chosen this can't be more than 60-75% of the individuals income.
Making a claim is simple (he says somewhat tongue in cheek!). You need to produce evidence of your income and your illness. For your illness this could be a sick note from your doctor (you can get a sick note whether self employed or employed) but more likely the insurer will write to the doctor for a report.
Unfortunately I can't recommend a product because I hate all insurance companies - they are only interested in two things taking your premiums and then finding a way (however elaborate) or wriggling out of paying the claim. But that's just my bitter experience!!