From the link:
Dressing up as “Tories” and “chavs” are given as examples of costumes to avoid, as these would breach the “class and political stereotypes” section of the guidance.
“Fancy Dress themes should also not be centred around political group stereotypes or the stereotypes of different levels of perceived class in the means to diminish their worth or validity,” the guidance says.
Reverse the outrage bus, it's not about Tories per se, and it's actually pro-Tory because they don't want to cause offense to those groups.