Two schools of thought on the issue:
1. The third person is best since it conveys an impression of objectivity, and at the same time making your argument seem more authoritive.
2. The first person is good because it brings you closer to your reader, and better conveys your own thoughts making your points seem more original.
I personally tend to stick to the third person when writing essays for my course. My tutor told me what I just mentioned above, and seemed to think that either is acceptable. Beyond this, I can't help you, especially with regard to the LNAT in particular. Have you thought of emailing them?