Hi Alison!
Welcome to planDisney.
It's always a pleasure to assist my fellow guests from the UK, and I'm happy to help with your question about park tickets.
One-day tickets for
Magic Kingdom Park are available, but unfortunately, you cannot buy these tickets from the UK. Once you arrive in the US you can purchase tickets at any Guest Relations window at all the Walt Disney World theme parks, and water parks, and at the Ticket Center at Disney Springs. You can also buy tickets online in the US at https://disneyworld.disney.go.com/admission/tickets/theme-parks/.
Prices vary, according to date and theme park, and as I don't know the exact date you want to visit these are the prices for your arrival date (including tax):
Magic Kingdom Park: US$142.71
EPCOT: US$126.74
Disney's Hollywood Studios: US$137.39
Disney's Animal Kingdom Theme Park: US$116.09
If you want to add the Park Hopper option this is available for an additional cost.
All prices are subject to change, and park hopping is subject to park capacity.
Please come back and visit us if you have any further questions, and you can
send me a direct question if you wish. We are happy to help you plan your magical holiday to Walt Disney World!
Sue