Government guidance on shielding and protecting those who are clinically extremely vulnerable from COVID-19 states that this group of people are strongly advised to work from home. If they are unable to do so they should not attend work for this period of restrictions. The full guidance is available here: https://www.gov.uk/government/publications/guidance-on-shielding-and-protecting-extremely-vulnerable-persons-from-covid-19/guidance-on-shielding-and-protecting-extremely-vulnerable-persons-from-covid-19.
The Government’s safer working guidance makes clear what employers should do to support clinically vulnerable and clinically extremely vulnerable workers. In all instances, employers must carry out a workplace risk assessment and take action, so far as is reasonably practicable, to ensure the health, safety and welfare of their employees. The Government’s safer working guidance can be found here: https://www.gov.uk/guidance/working-safely-during-coronavirus-covid-19.
If a worker has a concern about the safety of their workplace, they can raise this through their employee representative, trade union or directly to the Health and Safety Executive (HSE) or their local authority. Where HSE identifies employers who are not taking action to comply with the relevant public health legislation and guidance to control public health risks, they will consider taking a range of actions to improve control of workplace risks.
Individuals can seek impartial advice on their specific situation from ACAS (or the Labour Relations Agency in Northern Ireland). ACAS have also provided detailed guidance for those who are clinically vulnerable or extremely clinically vulnerable from COVID-19, which can be found here: https://www.acas.org.uk/coronavirus/vulnerable-people-and-high-risk.