Hi Daniel!
Welcome to planDisney.
Reservations open for
Bibbidi Bobbidi Boutique in Magic Kingdom Park at 6.00am (Eastern Time) which is 10.00am in the UK, until we change our clocks to British Summertime when it will revert to 11.00am, up to 60 days in advance. Unfortunately, at this time, guests who are UK-based can't make these reservations online (as we would for dining reservations).
You could try calling the Tours & Recreation freephone phone number on this
UK Phone Directory, or The Walt Disney Travel Company International on 0800 028 0778. Phone lines are open between noon and 10.00pm.
Also, it's possible that a Cast Member on the live chat feature on My Disney Experience mobile app "Chat With Us" might be able to help with this reservation for you.
Otherwise, as soon as you arrive in the US you will be able to make a reservation online or by calling US number (407) 939 7529,
subject to availability.
I know how disappointing this sounds, but I can only suggest that you keep trying if there are no appointments available. Cancellations can happen. Also, if you call into the Boutique when you are visiting Magic Kingdom Park, there's a chance they might be accepting walk-ins.
I wish you lots of luck with making an appointment, and maybe a dining reservation for breakfast, lunch or dinner at Cinderella's Royal Table
inside Cinderella Castle in Magic Kingdom Park could be a princess-themed alternative. You'll meet Cinderella to say hello and take some photos before going up to the beautiful dining room where several Disney princesses will visit you at your table while you enjoy your meal.
Please come back and visit us if you have any further questions and I'll keep my fingers crossed for your Bibbidi Bobbidi Boutique reservation.
Sue