Hi Rachael,
It's always lovely to be able to answer a question from a fellow Brit!
Mickey's Very Merry Christmas Party is a lot of fun - I have attended this Special Event several times in the past (with both friends and family) and it has always been enjoyed by all.
The party is held in the Magic Kingdom park on select nights in November and December from 7:00 PM - midnight. Separate admission is required for this Special Event and
tickets can be purchased right now from the Walt Disney World website. Your Special Event ticket includes entry into the Magic Kingdom park for the hours that the party will be taking place (7:00 PM - midnight).
On days that Mickey's Very Merry Christmas Party is taking place, Guests who have been visiting the Magic Kingdom park during the day with a regular Theme Park Ticket but do not have a ticket to Mickey's Very Merry Christmas Party, will be required to leave the park at 7:00 PM.
If I can be of any further help, please feel free to ask me another question.
Wishing you a magical holiday filled with memories which will last a lifetime!
Emma