From Lisa:

"The skip to content link should be the first tabbable link on every page, so I think it should appear before UIO. The link can be hidden generally but should be visible when tabbed to so it can be used by keyboard only users. Here’s an article on the topic and it goes over the method of hiding it until tabbed to (you can also see an example of how this work on that site): https://webaim.org/search/?q=skip%20to%20content%20link"

https://webaim.org/techniques/skipnav/