People who are unable to work because of coronavirus, and are eligible, can receive Statutory Sick Pay (SSP) from day one of their illness. This used to be paid from the fourth day of illness. The Department for Work and Pensions intends to legislate for this. This change applies from 13 March 2020.
People working in the gig economy or on a zero hours contract, may be entitled to Statutory Sick Pay. You can check if you are eligible on GOV.UK.
If you are not eligible to receive SSP you can apply for Universal Credit or the Employment Support Allowance. You may also apply for these if you can't work because you are self-isolating.