My situation is the follwing. I am a student in the UK and my girlfriend is a working professional. I am eligible for a "residential assistant" position at one of the university's student halls where I contribute some of my time and help out with students who may have issues and in return, they provide free accommodation. They are also happy for my girlfriend (who is not a student) to move in with me.
The catch is that she cannot have her official address there because accommodations with only students don't have to pay council tax. So they don't let non-students be registered as residents. However, they have told me explicitly that they're happy to let her live there provided she has another registered address.
My questions are
1) Is it legal to have a registered address somewhere without living there e.g. a friend's place?
2) Does the UK let you use a PO box address as your registered address?
She is a Swiss national and while free rent is great to have, we don't want to do anything illegal either. Is there a solution to my problem? Thank you!