Either get Euros in the US or use the ATM. If you get Euros in the UK then you are effectively paying two sets of commission, one on changing from dollars (your credit card currency) to pounds (the currency your card will be charged in), and on from pounds to to Euro. This could cause you to overpay by 10% or more.

If the ATM at CDG doesn't work (which would be very, very unusual) then there is a Bureau de Change (Exchange couter) in the terminal as well, where you could change a small amount of cash or get a credit card advance.

99 times out of 100 the ATM rate will be better than anything you can get anywhere else. From the US, all you have to do is ensure that your PIN number is exactly 4 digits long (and most already are).