Question: Was P.L. Travers left-handed, as Emma Thompson portrays her when she is shown writing?
Answer: I do not know the actual answer to your question. However, I would like to point out as a lefty myself that we often have to use our right hand for certain activities just due to the fact that left handed options are not readily available. Scissors and shears are a great example of this. Very often you cannot just switch them to your left hand and have them work. They actually have to be put together to be left handed to work properly. Also, many left handed writers are also ambidextrous. For example I golf right handed but bat left handed so the two swings don't negatively affect each other.
Chosen answer: It seems P. L. Travers was, in fact, right-handed. With just a bit of research, I found this YouTube video: http://www.youtube.com/watch?v=xeiEumLxTcM. At time reference 4:05, archive video shows Ms. Travers in her garden, holding a basket hooked on her left arm, and making clippings with a scissors in her right hand. Feeling convinced, I stopped, though I suspect further research (it's a six part biography) would yield other examples of P. L. Travers engaged in right-handed activities.
Michael Albert
Only problem with the assumption that travers was right-handed because she trimmed plants with her right hand is that there were no (to my knowledge) scissors for lefties. I was born in 1955 and I am a lefty who cuts right-handed, wear my watch on my left wrist, and made other adaptations due to the fact that left-handers were ignored, and travers was born over 50 years earlier.