All students in England and Wales have been issued with a 10 digit ULN (Unique Learner Number).
Don't worry if you are unsure what your ULN number is. You may register with AAT without a ULN number.
As you can see below in a screen shot of the AAT online student registration form, you are permitted to leave the ULN field blank. AAT will search for your ULN using other details you have entered in to the student registration form.
See the GET Support Portal article: AAT Student Membership and Subscription Fees for information regarding fees and a link to the AAT online student registration form.