Hi Claire and welcome to the Disney Parks Moms Panel.
It's always lovely to be able to answer a question from a fellow Brit!
When researching a similar question about a month ago, I was surprised to learn that guests from all over the world are able to purchase
Disney's Ultimate Tickets and book holiday packages to the Walt Disney World Resort through the
Walt Disney Travel Company, here in the UK.
You can read the whole of that
Question/Answer on the Disney Parks Moms Panel website.
As in your case you will be purchasing the tickets for your cousin and her partner, I see no reason why you can't purchase the Ultimate Tickets and have them posted out to your address in the UK. You should then be able to give her the Ticket ID Numbers to link the tickets in her
My Disney Experience account.
I would however recommend that you call and speak to one of the Disney Experts at the Walt Disney Travel Company on freephone 0800 16 90 730 prior to purchasing the tickets for confirmation that this will work.
Wishing you a magical holiday filled with memories which will last a lifetime!
Emma