You will need to have some class in order to represent a "free function". You are quite free in how to do that. What I usually do is to create a stereotyped class. And it would be ok to use «utility» for that. Anything else would work, but of course you need to document that in your domain.
Usually a stereotype is bound to a profile. But most tools allow to use freely defined stereotypes. Though that is not 100% UML compliant it is quite a common practice.
I have a number of free functions in a couple of Python modules and I need to create a UML Class Diagram to represent my entire program.
Can I represent a free functions in a Class Diagram somehow or do I need to create a Utility Class so I can represent them in my Class Diagram?
How to represent free functions in a Class Diagram Python