Common assault can result in a prison sentence, of up to 6 months, and/or a fine of up to �5000 but don't let that start you panicking! ;-)
It practice (particularly in the type of circumstances you describe) common assault is more usually a 'slap on the wrist' type of offence. A court might impose a community sentence (e.g. a certain number of hours unpaid work) but I doubt that they'd even bother doing that in your case. It would more likely to be just a relatively small fine (of, perhaps, �200), plus costs (perhaps �60) plus a �15 payment to the victim compensation scheme. A supervision order ( = 'probation') might also be imposed but I suspect that the court would see little point in doing so.
If you've been in your job for less than a year, an employer can fire you for any reason (or even in the complete absence of a reason). Thereafter, an employer could only dismiss you for something that occurred outside of work if
(a) your contract specifically allowed for such action ; or
(b) your actions resulted in a 'loss of trust' in your suitability for the post. (If you're a teacher, and the court case was reported in the press, an employer might reasonably believe that it was not possible for you to continue in your job. But if you stack shelves in Tesco there would be no such 'loss of trust').
Chris