You wouldn't have been charged unless the CPS had decided that there's a realistic chance of a successful prosecution, so they must be of the opinion that there's sufficient evidence to convince a court that you're guilty.
When someone is convicted of an assault charge, the court has to decide whether it falls into Category 1, 2 or 3. Based upon the limited information you've provided, it would seem likely that the offence would be classed as Category 3 (UNLESS, say, you used an implement to break the victim's nose or kicked him in the face - then it would probably fall into Category 2).
While a Category 3 offence under Section 20 can theoretically result in a custodial sentence, it usually doesn't. (A community order is the most likely outcome). However a Category 2 offence will always lead to a custodial sentence but, as long as it doesn't exceed 2 years, there's a reasonable chance of it being suspended.
See pages 7 to 10 here for more detailed information:
https://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf