Cover for long term sickness...
Income protection (previously known as permanent health insurance or PHI) is a policy designed to payout a specified monthly benefit to replace your salary in the event of you being unable to work due to disability or accident or sickness
There are long term policies out there, that usually kick in after a least 12 week to cover long term illness and require full underwritting and there are specialist policies for the self-employed.
The other thing to consider - try and tie up with someone else in your field. I have started recommending clients running Ltd companies to do this, especially if they are IR35 borderline companies.
Initially you do it so you can go on Holiday. They provide holiday cover for you, and you do the same for them. You help (and pay) each other for a couple of weeks a year for covering each others work/contracts etc...... Sign a simple confidentiality etc. agreement so there are no conflicts.