UK train drivers were hired by an Australian freight company about 15 years ago. The company's Conductors and others lower down the line of promotion weren't very happy about being leapfrogged.
Train Driver is not currently on the list of jobs where there is a shortage of skills in Australia or New Zealand, so you would now have to get in on probably your partner's skills, hope for a trainee driver job to come up, hope to be successful, and then climb the seniority ladder to get the depot you wanted to be at.
It takes long enough to get the depot you want to be at in the UK sometimes!
If you could "just drive trains abroad", how come a separate rule book is needed to be learned for UK Eurostar drivers to be able to work the Belgian services (as well as the French one)?
It's always been a job where there are differences in each country.