The Health Protection (Coronavirus, Restrictions) (Self-Isolation) (England) Regulations 2020 are now in effect (Monday 28 September 2020).
They set out mandatory periods for self-isolation, and a duty to notify the Secretary of State of the names of people in the same household as anyone who has tested positive for Covid-19.
The important element for employers is Regulation 7. This makes it an offence for an employer to knowingly permit a worker (including an agency worker) to attend any place other than where the individual is self-isolating. This includes individuals who are required to self-isolate because they live with someone who has tested positive. So if an employer knows a worker has tested positive (or lives with someone who has tested positive), it is now responsible for stopping the worker from working (unless they can work from home). Any employer who fails to do so will face a fine, starting at £1,000.
There is also an obligation on the worker to tell their employer that they are self-isolating (reg 8). Any individual who breaches self-isolation will, normally, commit a separate criminal offence (see Reg 11).