Maybe the screener was annoyed at being asked to speak English dependent on the OP's tone and possibly the impression that the English travel and expect everyone else to speak English.

I've come across English people prejudiced against Germans but this was primarily older people and due to memories of WW2.

It sounds like everything escalated after the initial interaction.

Not sure what you can do other than to file a complaint. Also not sure why the OPs thinks the entire German population is not racist. Just like every population I'm sure there are plenty of Germans that are racist.