a uk senior civil servant has resigned because of Patels bullying
Perhaps that should read "because of reports of Patel's bullying" as it appears that Rutnam has not witnessed it himself but is relying on the claims of others, according to his resignation speech recorded by The Independent
“I have received allegations that her conduct has included shouting and swearing, belittling people, making unreasonable and repeated demands – behaviour that created fear and that needed some bravery to call out.
“I know that resigning in this way will have serious implications for me personally – the Cabinet Office offered me a financial settlement that would have avoided this outcome.
“I am aware that there will continue to be briefing against me now I have made this decision, but I am hopeful that at least it may not now be directed towards my colleagues or the department.
“This has been a very difficult decision but I hope that my stand may help in maintaining the quality of government in our country – which includes hundreds of thousands of civil servants, loyally dedicated to delivering this government’s agenda.
“I will make no further comment at this stage.”

If, as he says, "The Cabinet Office offered me a financial settlement" then it looks as if he was on his way out anyway, and has jumped before he could be pushed. As such I doubt he will be successful in claiming 'constructive dismissal'.