UK Immigration Law
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As your passport has expired it is no longer a valid document and therefore there is no obligation on any airline or country to accept it for travelling or for that matter any other purpose. Although an airline or foreign country could in their discretion allow you entry you risk being refused entry or left being stranded in Germany until you somehow resolve your return travel. The safest thing for you to do is to therefore obtain an emergency travel document given the shortage of time that you appear to have.
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