Unless it's racially motivated, the maximum penalty for common assault is one of 26 weeks imprisonment. See pages 189 to 192 here:
https://www.sentencingcouncil.org.uk/wp-content/uploads/MCSG-April-2017-FINAL-2.pdf
The maximum penalty for threatening behaviour is the same. (See page 75 of that document).
Assaulting a group of people would result in separate charges, possibly leading to a cumulative punishment. Threatening a group of people simultaneously though would only be a single incident of 'threatening behaviour', and so could only lead to a single charge.
Harassment that puts people in fear of violence though (which would need to involve more than a single threat on one occasion) can lead to a prison sentence of up to 5 years (or of up to 7 years if it's racially motivated). See pages 63 and 64.
Threats to kill can result in a prison sentence of up to 10 years. (See page 96).