What was the reason of denial? As you quote Brexit, I guess you had a one way ticket and non-EU nationals need a RT ticket to be able to travel to DE. Right?

Then the easiest workaround was to quickly purchase a refundable FRA-LON ticket for the following week and refund it once you once you reach DE.

Have you checked Timatic? If it does not mention the rent agreement workaround you might be out of luck as most airlines follow whatever Timatic says.