Hi Christine,
How lovely that you will be able to spend time with your family from the UK at Walt Disney World this year!
I would recommend that you cost your trip up as you normally would from home and then as a comparison, price up the same trip via the
UK version of the Walt Disney World website. (If you are re-directed to the USA version, scroll down to the bottom of the home page and where it states "Select an International Version" click on "United Kingdom: English" from the drop down box)
I wouldn't necessarily say that it will be less expensive to book from the UK. My family have booked our entire trip through the USA version of the Walt Disney World website in the past as it worked out cheaper.
If you need any more help with booking or planning your trip, please visit us again on the Walt Disney World Moms Panel and ask me another question.
Wishing you a magical holiday filled with memories which will last a lifetime!