As we have two girls we have discovered that the British dresses are much better quality than the US dresses and they vary quite a lot in design as well. For my daughters 7th birthday last December I booked both girls in for the basic bibbity bobbity boo boutique makeover and packed two new dresses to surprise the girls with on the morning. For the birthday girl I bought the new Elizabeth Swan dress that is Gold in the UK but only comes in White in the US (and is prety naff by their own standards) this caused quite a stir as the staff had not seen it before. We also got stopped quite a few times that day and asked where to purchase the dress to which we had to say you can only get it in Woolworths in the UK lol.
Also another couple of tips, I decided to save a few dollars and not have their nails done as I had read that the plastic nails are not very good and fall off after a few minutes, also my 4 year old bites her nails and they are miniscule so there is literally nothing there to paint! also make sure they know that your daughter is celebrating a birthday as they give a different sash to the normal one. Also even if you are not buying the photo package the photo pass people are in the salon and take lots of snaps of the girls being made up so if there is a good one you can always buy it online afterwards.
Have a lovely time!