The law has not changed.
The Health Protection (Coronavirus, Restrictions) Regulations 2020 never explicitly instructed Flying Schools to cease business.
However guidance from the CAA and DfT did. This is guidance and not legislation.
2 meter "social distancing" is appended with "if practical". For certain jobs and businesses 2 meters separation will never be practical.
WHO guidlines for workplaces are very measured; emphasising good hand washing and cleaning protocols. They emphasise not coming into work when feeling unwell. There is also detail on protecting and providing information to the vulnerable.