yes, when I was in business on my own I took out PHI (Permanent Health Insurance), and once had the misfortune to make a substantial claim. I was off work for about a year.
Policies and costs vary (ask a few Assurance companies) but what you want is a policy which pays out if you are unable to carry on your normal occupation (e.g. if you are a ballet dancer, it's no good being told, when you lose your legs, that you are still fit to work in an office).
It costs less if you specify a waiting period, e.g. 3 months or 6 months, because most people are better by then, so they don't have to pay out.
I had two policies, one with a 3-month and one with a six-month delay. I had in mind that I might get MS or something and be unable to work for the rest of my life.
For short illnesses it is more economical to just tuck some money under the mattress to tide you over.
The premium will be a percentage of the benefit. e,g, pay £20 a month, for a benefit of £500 a month; or £40 a month for a benefit of £1,000 a month (figures are made up). You can get inflation linking so that the benefit, and the premium, increase each year. If you become disabled, you no longer have to pay the premium, but with an index linked policy the benefit will increase for the rest of your life, or until you are fit. The policy will probably end when you hit 60 or some other planned retirement date. The insurers will insist that you are medically examined before you take out the policy, and at intervals during a claim.
Beware of salesmen who make their money out of selling you an expensive policy. I would avoid a Critical Illness or an Accident policy, because they only cover you for certain things, and with my luck I'd be insured for 100 events, and go down with the 101st
There are also a few Friendly Societies and Industrial Branch policies, AFAIK they are for very small amounts. i was once with the Holloway Society, I think Hearts of Oak do a scheme. Your union might have a scheme, but it might not be good value if they take a fat commission.
edit
found this http://www.thisismoney.co.uk/money/bills/article-1583955/Permanent-health-insurance.html