You have to cancel your account by conventional snail mail! Haha.
From their FAQ:
You have the right to remove your personal information and preferences. In order to do this, please write […] including the following information:
Your name,
Your address and/or…
The e-mail address you set up the profile with.
Post your letter to the following address:
United Direct
c/o Kitbag Ltd
Greengate
Manchester
England
M24 1FD
URLS:
http://sotre.manutd.com
Please note that AccountKiller.com is not associated with the business listed on this page. AccountKiller provides easy-to-understand instructions and guides on how to cancel charges and relationships from the list of businesses on our site, we have no affiliation with these businesses.