Here is the question, is her anxiety enough to consider it a disability. Does it impair her is a large enough way in day to day activities. And has this anxiety issue been long term. I think that it is possible for anxiety to be severe enough to be considered a disability; however I would also say that it would be uncommon.
Is your daughter's anxiety preventing her from doing things a normal child her age would be doing? Does the fear prevent her from being able to act. Is her performance on tests significantly different when she sits alone, compared to when she sits in class.
Clearly it is not a small problem as the school has been making adjustments for her. As her taking tests in isolation creates extra work for them, I would think they feel her anxiety issues are in the abnormal range. My advice would be that when the time comes, assuming your daughter has not gotten over her anxiety issues,apply for her to sit on her own for the eleven plus, using the fact the school does this for testing as part of your evidence. Provide as much information as possible. It is highly likely that without outside agency reports it will get rejected; however, seeing as she has high CAT scores and the school normally makes adjustments for her, if all goes wrong with the eleven plus, the fact you requested and were rejected for her to take the test in isolation could help your case in an appeal/review. This is because you would have shown that there was a known problem with her exam taking before the eleven plus and that you aren't just saying it after the fact.
I would ask the school now if they feel her anxiety warrants a referral to an educational psychologist. It may be a short term thing, it may not be. However, the fact that if it continues, you have evidence of her problems dating back to year 4, it will give weight to your arguments.