Mexican travelling to England (Mexico City)

My girlfriend and i are going to England and Europe for a holiday . I have a working visa but she doesn t . I was just planning on working for a month or so before we backpack around Europe . We re a bit concerned about how she will go at immigration , we ve heard mixed views on the topic . Does anyone know what documents she will need and what ticket she will need ie. Return flight or ticket to another country .
Cheers