They can sack them, whether it is subsequently determined to be a fair dismissal is another matter and only if they have 2 years service could they go to a Tribunal. The government has done nothing to extend employment rights as a result of Coronavirus beyond some change to SSP.
If an employee turns up for work with a fever/cough surely they are obliged to send them home on full sick pay? My belief is that an employer cannot send an employee home. Easy solution - cancel HS2 and divert the funds to keeping businesses afloat.