Ask most households what would happen if the main earner could not work for a year, and the answer is a pause. People insure the car and the phone and the holiday. Far fewer insure the thing that pays for all of it, which is the income itself.
The gap is usually hidden because people assume something will catch them. Employer sick pay, savings, the state, a partner's wage. Each of those helps, but none of them is built to replace a salary for months on end, and the shortfall between what you think you have and what you actually have is the cover gap.
What sick pay really covers
Statutory sick pay is a modest weekly amount and it runs out. Many employers pay more than that, but often only for a few weeks or months, and the generous schemes taper to nothing sooner than people expect. It is worth reading your own contract rather than assuming. The honest version of "my employer covers me" is usually "my employer covers me for a while".
Income protection is the product built for the rest of that period. It pays a monthly amount if illness or injury stops you working, and it keeps paying, within the terms, until you recover or reach the end of the policy. It is not a lump sum for a diagnosis; it is a replacement wage while you cannot earn one.
The two settings that decide the cost
Two choices move the price more than anything else. The first is the deferred period, which is how long you wait after stopping work before the payments begin. If your employer pays you for three months, starting the policy at three months instead of day one cuts the premium sharply, because you are only insuring the part you cannot cover yourself.
The second is how long the policy pays once a claim starts. A short-term version pays for a year or two per claim and is cheaper. A full-term version pays until retirement if needed, and costs more because it does the harder job. For a household that depends on one income, the full-term version is usually the one that matters, even if the budget only stretches to a smaller amount of cover.
It is not a lump sum for a diagnosis; it is a replacement wage while you cannot earn one.
How much to cover
You cannot usually insure your whole salary, and you would not want to pay for it. Aim to cover the essentials: the mortgage or rent, the bills, the food, the things that do not stop when your income does. Getting the monthly amount roughly right and the deferred period sensibly set does more good than chasing a perfect figure you cannot afford to hold.