IIRC, what you British call sirloin is what we call NY Strip, which is a pretty good steak. What we call sirloin here is what I call dog food. The one recipe calls for a steak that is an inch thick, which is decent, and a lot thicker than what I've seen British chefs use. If you are going to use a convection oven, AKA and air fryer, you will need at least an inch-thick cut of beef, or by the time you get a nice crust, it is going to be well-done in the middle.
Again, I think an air fryer is the wrong tool for the job, like using a hammer to drive a screw into a piece of wood -- yeah, it'll work, but why would I do it?
CD