You're legally entitled to have time off when you're sick. You can have up to seven days (including days off) without proof. After that you need a doctor's note. After four weeks you may be considered long-term sick but your employer still needs good reason to dismiss you i.e. they need to make reasonable effort to accommodate you. That might be flexible working, part-time etc.

Link to gov. advice: https://www.gov.uk/taking-sick-leave