Hi
I have documents showing details of my mum who was interned and her sister who wasn't obtained from Bad Arolsen https://arolsen-archives.org/en/ 
Although I cannot suggest a legal reason for internment I understand that everyone who was to be interned was arrested around May 1940 due to an outcry in the British press about working alongside foreign spies. My poor, innocent,  quiet mum was put in Holloway until they were able to move her to the Isle of Mann. Having said that she was looked after and fed well and had freedom within the town. Sorry I cannot help more.