I'm not familiar with this particular assessment, but the school psychologists I've worked with in the past would treat any cognitive assessment given before, say, third grade, as provisional. The endurance issues you describe are one of the reasons for that.
And as far as what the benefits are, I'm not entirely sure why you are testing, so that question is difficult to answer. I'm going to recommend a book instead. 5 Levels of Gifted: School Issues and Educational Options by Deborah Ruf. I would think this book could help you narrow down to, say, two levels, even if only by comparing your behavioral observations with the ones in the book. And that should help you get a clearer picture of the educational needs of your daughter.
If you find a program in the UK, they will probably have their own requirements for testing. I'd cross that bridge when I come to it if I were you.
