If you’re unable to work due to illness or are self-isolating due to Corona virus, and your employer doesn’t offer contractual sick pay, you can claim Statutory Sick Pay (SSP) if you usually earn at least £118 per week before tax. For the next 3 months at least, you can claim SSP from the first day of not working. Your employer pays SSP and you should tell them immediately, and at least within 7 days, that you need to claim it. You normally have to provide a GP fit note after 7 consecutive days of illness but at present you only need to provide a note from NHS 111 online
If you qualify for SSP, you also qualify for some Universal Credit monthly payments towards rent costs and for Council Tax Reduction. Use the benefit calculator at the Turn2Us website to calculate how much you qualify for and how to apply for it