I promised to post these links when at the PC. I know the question is now closed, but I am posting them for the sake of completion (and because I promised!). Also in case they are useful in future.
Note "serious insubordination" as included as a form of gross misconduct on p.32 of this link:
http://www.acas.org.uk/media/pdf/9/g/Discipline-and-grievanc...Also, end of first paragraph of section 2 of this document:
https://www.lawdonut.co.uk/business/employment-law/disciplin...As mentioned previously, I can only find
serious /gross insubordination as a 'sub-section' and, unless an employee is wholly unsuitable (and probably shouldn't have been hired in the first place!), it should take quite a bit of doing to get to this point!