It should only offer uppercase tag names if your typed tag starts with uppercase characters.
There's the following code in $VIMRUNTIME/autoload/htmlcomplete.vim
:
if &filetype == 'html' && exists("uppercase_tag")
So one workaround would be switching the filetype to xml
(but then it doesn't know about all HTML tags, only existing in the document), or you can modify the script and put your copy to ~/.vim/autoload/htmlcomplete.vim
, so that it will override the default one.