Hi Emma,
It's lovely to see a question from a fellow Brit!
1/ The best way to book tickets for
Cirque du Soleil La Nouba from here in the UK would be to book them
online.
2/ Although I personally have never used it, I do believe that AA members in the UK are eligible for some discounts in the Orlando area. (You can find out more about this and see a list of where these discounts are available from the following link: http://ww2.aaa.com/travel2/sycs/memberDiscountsSearchResults.xhtml?dt=1332702767027&appcontext=aaaDotCom&office=998&app=aaaDotCom&location=PLACE%7CAAA347%7COrlando%7CORL%7CUS&association=AAA&recache=&club=700&lang=EN&memid=&conversationId=1683)
3/ I can't imagine that you would have any problems at all in using a prepaid Mastercard to make purchases whilst at Walt Disney World. We have used prepaid travel money cards from the Post Office during previous trips without any issues whatsoever.
Should you need any other help please visit us again and ask me another question.
Wishing you a magical holiday filled with memories which will last a lifetime!