Hi Georgina!
Welcome to planDisney.
It's always a pleasure to assist my fellow guests from the UK. Thank you for sending us this question.
Unfortunately, it is not usually possible to book any experiences at Walt Disney World that require pre-payment in advance from the UK. This includes
Savi's Workshop at
Disney's Hollywood Studios. I recommend that you try the Chat feature on your My Disney Experience app, as sometimes they might be able to make reservations for you
(but not guaranteed). Tap the "≡" in the bottom right-hand corner of your screen on My Disney Experience, then scroll down to "Help & Feedback", then "Chat With Us".
I spoke with a very helpful Cast Member who confirmed that they can make reservations for UK guests. This is always subject to change, but I hope you will be able to make your bookings via Chat when it comes to your 60-day mark. Perhaps you could give the Chat service a try beforehand, maybe to confirm the times the bookings open - just so you are ready on the day.
I hope you have success with the chat feature, as I do understand how disappointing this feels.
Otherwise, these tours can be booked when you arrive in the US, either online or via the My Disney Experience app. The phone number you quoted is the 'Tours & Recreation' number in the US, so you would have to precede it with 001. If you are calling once you arrive in Florida, you don't need to add the '001' - that's just for calling from the UK (or anywhere outside of the US).
Please come back and visit us if you have any further questions - we are always happy to help plan your magical holiday to Walt Disney World!
Sue