Hi Bobbi,
You can absolutely make your Advanced Dining Reservations (ADRs) from right here in the UK!
If you want your reservations to show up in the 'My Reservations' section of the Walt Disney World website or have included the Disney Dining Plan as part of your package, you will need to contact the travel agency with whom you made the booking to request your US confirmation number.
Once you have obtained the US confirmation number you can start to make your ADRs
HERE (Be sure to click on where it says 'I want to use my dining plan to book reservations' and enter your US confirmation number, surname & arrival date before making your reservations)
Otherwise, you can go ahead and make your ADRs online straight away. You will be asked to enter your phone number & e-mail address when making the reservation and will receive an e-mail which contains your confirmation number.
Wishing you a magical holiday!