if you enter a country whose citizenship you have you must always enter with the travel document or proof of citizenship of that country. no exceptions. that is a global convention. using a foreign document is most likely a violation of the law. (there may be exceptions in some countries that don't issue passports abroad).

that waiver document is ridiculous though. what does it cost to get a new passport at a british embassy? as a german i can get a temporary passport within a day at any german embassy for about 30€ or 60€. enough to travel back home.