'Assault by beating' is an alternative wording to 'common assault'. (i.e. it's still an offence contrary to Section 39 of the Criminal Justice Act 1988 and carries the same range of penalties). As such, it's the least serious of possible assault charges.
Although 'assault by beating' can theoretically lead to up to 6 months imprisonment, custodial sentences for the offence are extremely rare. Such a sentence is only available for a 'Category 1' offence, where both 'harm' and 'culpability' are seen as 'high'. (e.g. an organised attack by a gang on a person with learning difficulties). A community order is a far more likely outcome in most cases.
See pages 189 to 192 here:
https://www.sentencingcouncil.org.uk/wp-content/uploads/MCSG-April-2017-FINAL-2.pdf