You're right about them not being law enforcement(See https://www.mi5.gov.uk/home/about-us/faqs-about-mi5/is-mi5-the-secret-police.html https://www.mi5.gov.uk/h

You're right about them not being law enforcement(See https://www.mi5.gov.uk/home/about-us/faqs-about-mi5/is-mi5-the-secret-police.html https://www.mi5.gov.uk/home/about-us/faqs-about-mi5/is-mi5-t... for example) but the level of collaboration between them is very close.