The middle option nobody costs out: four boxes and a relative's loft.
Once you have applied the rule above, what is left is usually small enough not to need a commercial unit at all. Mine is documents, photographs, two boxes of things I could not part with, and a bag of winter clothes. It costs nothing, it is somewhere I visit anyway, and it removed the monthly reminder that was making me feel bad every time the payment went out.
Two practical things if you go this way:
Do not do it without asking properly, and do not let it be open-ended. "Until I get a place" is how a favour becomes a grievance. Give a date, and if the date moves, say so.
Make a list of what is in each box and photograph the contents. The reason you cannot decide about your unit right now is that you do not actually know what is in it. Whatever you keep, know what it is — a list on your phone means you can make this decision in ten minutes next time instead of turning it into a trip home.
And on your fear about furnishing a flat from nothing: it is genuinely a bad week, but it is one week and it is cheaper than you think second-hand. It is not two years of rent on a room for objects.