I think a combination of the both. Make an effort but don't look like you emptied topshop! If you're going for the design BA don't go in a black suit - boring! Have a good handbag would be my recomendation, with a cool outfit. I personally wouldn't wear jeans. But a nice trendy outfit that smart but not too much.
The questions they asked me were:
1. Who inspires you the most?
2. Why?
3. Who do you think has it completly right on the high street - why?
4. Which collection has had an impression upon you the most? i.e. f/w 2004 Alexander McQueen (then describe something that made its mark on you)
5. Any specialist talents eg. embroidery, handsewing, talioring etc. How would you incorporate it into your design work.
6. Any fabrics you really like to work with
7. Work you do in your spare time (eg. scrap books, photography, handsewing)
8. Any experiments you like to do eg. smocking onto alternative fabrics like paper touch cotton etc
Its pretty obvious stuff, the above is for design courses mainly. For the BA, they will really come down on you on extra curricular stuff and your knowledge of designers (name some indie ones too (Willow, Sass and Bide, Lisa Ho etc- they love that) It must get a bit dull hearing "Oh I just looove Gucci" 30 times a day!
Oh yeah, talk about the previous Fashion Weeks (NY, London and Paris and say what stood out to you the most)
I hope all this helps, its what they asked me.