Hi there
Im currently a sales adviser in an o2 shop so can give you a bit of an insight.
The first piece of advice i can give is be confident. Obviously this accounts for any interview but it's especially important in a sales adviser role as you will have to speak to everyone and anyone if you get the job and if you hesitate during a sale it gives a bad image of yourself, the shop and the company. however ensure you dont come across as arrogant as no one likes that, just be confident in yourself and you'll do fine.
Also they will probably ask you a few questions about the business (for example they asked me in my interview why o2 was awarded the iphone contract from apple a year before the other competitors, the answer to which was something along the lines of "because of the way o2 looks after its customer's and the quality at which o2 provide its service")
However they did not ask me to do a presentation so i cant help you there really so i aplogise. But following on from before i suggest you just pick a subject your confident in as from the sounds of it, they are not particularly looking at the subject you choose (but obviously dont choose something that looks bad like drink or something) but rather the way in which you present yourself so try your best to stay calm, remain confident and try not to hesitate + use good language.
Hope this helps mate and if you have any further questions just let me know and ill be more than happy to help
Aaron.
P.S. let us know how you do