New regulations come into force today (28 September 2020). They set out mandatory periods for self-isolation, and create various offences. The important one for employers to note is that under Regulation 7 it is now an offence to knowingly permit a worker (including an agency worker) to attend any place other than where they are self-isolating.
This includes individuals who are self-isolating because they have tested positive themselves, or those who are self-isolating because they live with someone who has tested positive. If an employer knows someone has tested positive, or lives with someone who has tested positive, the employer will now have an obligation to stop the employee from working (unless they can work from home). An employer who fails to comply can face a fine, starting at £1,000 (with further increased fines for repeat offences).
For details of The Health Protection (Coronavirus, Restrictions) (Self Isolation) (England) Regulations 2020 see here: