Aaagh! I've just typed out a detailed answer on the other thread and it's disappeared! Let's try again!
'Assault by beating' is simply an alternative wording for 'common assault', which is the very lowest level of assault charge.
If you'd simply thrown your drink over the guy the offence would be classed as 'Category 3' but, because injury occurred, a court would probably class it as 'Category 2'.
Given the circumstances you describe, the lowest sentence within Category 2 would probably be applied, which is simply a fine (plus costs and a 'victim surcharge'). The worst that could happen would be a Community Order (i.e. unpaid work) but I doubt that would be made. The events that you describe clearly don't fall into Category 1, so there's absolutely no way that a prison sentence could be passed.
Face up to it: throwing a drink over someone is a criminal offence (however much that person might have been going out of his way to annoy you). You'll probably end up a bit poorer because of it but there's nothing really to worry about.