MI5 have a very long, and proud, history of distinguishing between political enmity and "subversive" activity. The history of "entryism" during the Cold War is well documented, and MI5 have declassified information about what they did and did not do. Basically they helped protect the Labour party from infiltration from Communist sympathising individuals who wanted to overthrow Parliamentary democracy. They did not spy on merely left wing individuals.

We're in an age of leaks, and there have been whistleblowers from the UK intelligence services. If MI5 were asked to spy on Corbyn you'd probably hear about it in the resignation letter of the head of the service...

Personally I do think Corbyn is a threat to national security. Not in a sense which he is aiming to destroy the state or country, but that his naive and inconsistent world view would embolden our enemies and be more likely to cause conflict. His views are one which he is entitled to have without state surveillance, he's entitled to stand for public office and entitled to be brutalised by his political opponents for his previous positions, especially on foreign policy.