As for care homes making their own decisions, perhaps those that have closed to all visitors will review the situation as time goes on and realize they need assistance from relatives with personal experience.
I guess that homes that took the decision to stop all visitors prior to the issuing of the new guidance might well now review their actions. As the guidance states, the wellbeing benefits to residents from seeing friends/relatives also needs to be considered - it's a balancing act. Mum's care home is still open to visitors so I'll continue to go as normal unless I start feeling unwell. They have loads of posters up about hand washing, and plenty of sanitiser also available, so just have to hope that other visitors are also sensible and take the necessary precautions.