Hello Jennifer!
Welcome to planDisney! Whether it is a week or just a few days in Walt Disney World Resort, there is plenty of fun to be had and snacks to enjoy and I am glad you are making the best of it!
As a guest from the UK, the best way to purchase a two-day ticket to the Parks is to wait until you arrive in the United States. I suggest buying a 2-day ticket as you can use each day at any Park you like and it's a better value than buying two one-day tickets. You are correct that UK guests only have the option to purchase a
7-Day or 14-Day Magic Ticket. There is a
2-day ticket option for residents of the EU that you may be able to purchase but I am not sure if the site will allow you to finalize payment. (Always worth a try!) Regardless, you can still purchase the ticket you want when you arrive by heading to the
US-based ticketing site when you arrive. These are date-based tickets so you won't need theme park reservations and they can be purchased last-minute with no issue. The week you are arriving is a holiday week for many in the US but it usually doesn't lead to Park capacity issues. Just in case, I recommend that on the day you wish to visit
Magic Kingdom Park arrive early to secure entry. If the Park gets too crowded then guests can be prevented from entering even with valid tickets.
I hope this helps and you have a fabulous time!
Happy Planning!
-Nimmy