"The guarantee does not apply when late delivery results ... from events beyond our control ... or the exercise of any lien in accordance with our Terms and Con

"The guarantee does not apply when late delivery results ... from events beyond our control ... or the exercise of any lien in accordance with our Terms and Conditions of Carriage." If you can void your guarantee based on "forces outside your control" in something like shipping, where you're at the mercy of traffic, weather conditions, and human error, it's not really a guarantee. The UK is a substantially easier country to ship in logistically than the US. I don't doubt that Royal Mail's guaranteed delivery works.