Can you show a survey of actual threats which supports the thesis that left-wing extremism is as insignificant as you claim it to be? From what I am able to see there is not that much difference between the two.

Here's a recent report on left-wing extremism in the United Kingdom:

https://assets.publishing.service.gov.uk/government/uploads/...

I have not found a similar study on far-right extremism but there is a report giving an overview of the far right:

https://assets.publishing.service.gov.uk/government/uploads/...

Reading these reports shows that political violence currently is at a low level in the UK but that it is on the rise internationally which has the potential to lead to an increase both on the far-left as well as the far-right side.