From the limited information available, it looks like things are stacked against the person who is required to isolate by the employer.
You can ask your employer to allow you to take the sick period as holiday leave, and if he / she refuses, you can ask for unpaid leave - clearly if the employer demands you isolate and will not cover this with sick pay, then what the hell else is it other than unpaid leave!
You can also, under certain circumstances, be dismissed if you are off work with COVID! But it has to be a 'reasonable' cause for such dismissal...
... seems fair doesn't it 