Thanks for the link. I did investigate the issue:
User Agent for IE11 on Windows for ARM is: Mozilla/5.0 (Windows NT 6.3; ARM; Trident/7.0; Touch; rv:11.0) like Gecko
It can be read as (OS Version, Build, Engine/Version, Touch, Client Version)
It is the same string as IE 11 on Windows x64 except that ARM is replaced by WOW64.
This particular website does check if there is ARM in the user agent string and then provides mobile version. It does not explicitly check if it finds WOW64, so just removing ARM gets you the desktop site. Unfortunately you cannot permanently change the User Agent String but only temporarily as far as i am aware.
So best bet is emailing the web-site provider to change this behaviour, because assuming ARM->mobile is certainly wrong. Instead they can figure, that because the OS is Windows NT 6.3 (aka Windows 8.1), it is a desktop class browser.