Pblakeney
Squire
Not necessarily true, many usually have some clear experience professionally or have served in Junior roles with a link to their cabinet post. Look at Chancellor over the last couple of decades.
Rachel Reeves - banking
Kwasi Kwarteng - financial analyst
Rishi Sunak - banking/analyst
Sajid Javid - banking
Phillip Hammond - Shadow Treasury Chief Secretary
George Osborne - shadow Treasury Secretary/Shadow Chancellor
It's certainly not unheard of, but usually someone getting one of the big Cabinet posts does have some form of credentials that makes it apparent as to why they have been given the role.
I have to say that imo holding a shadow position does not count as experience.
Does James Cartlidge really have direct relevant experience in Defence?