UltraEdit does not specify itself as HTML source viewer in Internet Explorer. It just adds itself to the list of source viewers.
All editors capable editing HTML files I ever installed (Frontpage, Dreamweaver, Microsoft Word, ...) has added itself to the list of source viewers. So if 1 or more source viewers are already registered, adding UltraEdit to the list will not change anything.
The real problem is that Microsoft has not registered Notepad as source viewer and simply uses Notepad, if no source viewer is registered. So the first program, which registers itself as possible source viewer for HTML files is automatically used by IE as source viewer. The editors have to add itself to the list of source viewers, because no user can do it manually (except the user is an expert and knows how and where to modify the registry).
See also UltraEdit takes over Windows Notepad as source viewer in IE
Did the installer of UltraEdit not asked you about setting UltraEdit as IE viewer for webpages?