Plea: Guilty
Irrespective of the amount of provocation (and your former girlfriends exaggerations) , your description of events confirms that you've committed common assault.
Likely sentence: Conditional discharge (or possibly a fairly small fine)
Effect on job: Probably nil if you speak to your boss
before the court case, explain the circumstances, and tell him/her how concerned you are about the possible impact on your employment. See here:
http://www.theanswerbank.co.uk/Jobs-and-Educat ion/Question538502.html
Effect on travel plans: Probably nil if you only intend to travel as a tourist. Very few countries have rules which might bar UK passport holders, with criminal convictions, from entering for the purpose of tourism. The USA would, according to the rules on their embassy's website, require you to apply for a visa but common assault is not a crime of 'moral turpitude' so you'd probably be granted one. (In practice, most people simply 'forget' about their convictions and enter under the Visa Waiver Program. The USA have no direct access to UK criminal records, so their immigration service wouldn't know about the assault). Canada would technically require you to wait until you were 'rehabilitated' but they don't carry out any checks. Similarly, Australia doesn't carry out any checks if you enter with an Electronic Travel Authority.
Chris