That was rather harsh then. An assault upon a person acting in the course of their duty is an aggravating factor but its not worth a suspended sentence on a first offence.
In any event, if you are not on a suspended sentence now and you have no other violence than this on your record, and you are charged with a common assault then you really should escape custody.
There is only one aggravating factor here and that is that he did suffer injury even if it was only minor. I suppose they may not like the fact that it happened when you were under the influence of alcohol which does seem to be a theme of your antecedence from what you say but I don't think amounts to an aggravating factor. If there's only one aggravating factor then this should not cross the custody threshold unless there is concern that you will not comply with a community order or it will not protect the public.
If you've completed your previous orders that will be helpful. I suppose it depends how the alcohol order you had previously went. If you were not on an alcohol order then you may need to expect that type of order on this occasion. In any event, if you've done your community orders they would struggle to justify custody.
Of course, that does depend upon you being charged with common assault not ABH.
In future though, we never admit things at the police station. If you'd made a no comment interview then you may well have walked away with nothing here.