What is the most important thing employers should do from a health and safety perspective?

Conduct risk assessments! Your RA must cover every foreseeable risk arising from a return to the workplace, including the impact of reduced staff levels and any operational/administrative changes necessary to ensure social distancing.

Appropriate steps should be taken to manage and mitigate identified risks. Where this is not possible, businesses need to decide whether certain activities are necessary for the business to operate or if they can be temporarily put on hold.
Keep a close eye on the comprehensive Government guidance: https://www.gov.uk/guidance/working-safely-during-coronavirus-covid-19

In particular focus on social distancing and workplace health measures. This guidance will evolve over time and you will need to be sure that your organisation is sticking to it AND reviewing and updating its risk assessment.