Shadowing
I too looked for this route many years ago, and found it a dead end. I don't think it's done in the UK, and as you referred to security clearance it's unlikely it could ever be a goer.
The only thing close would be a cadet ship where you work in other departments and eventually finish up in the right hand seat. Jet2 offer this to a few people, and highland and Atlantic used to. Some others may do to but I don't know who.
Hope that helps.