Shouldn't the instructor in this scenario try to avoid a deadstick landing?
if that was a real event, shouldn't Captain Townsend (as the instructor) interrupt the relaxing gliding with a warning? Something along the lines of &quot;excuse me, sir, we're losing altitude, if you want me to restart the engine we have to do it now&quot;, or even &quot;I'm restarting the engine now&quot;. I mean, shouldn't he be checking the limit altitude for restarting the engine?

This question appears to be predicated on the idea that the instructor had no plans to land the plane with the engine turned off. Having watched the scene, I'm far less convinced that this is the case. In fact, it's not until Philip remarks that the ground is coming up that the Captain tells him they'll be landing deadstick.
In the series, this instructor featured is a renowned flying ace with huge amounts of combat flying experience including a number of forced landings and ditchings.
Bob Hoover (who you could argue with cut from a similar cloth) regularly displayed and demonstrated with the engines turned off, up to and including landing in that configuration.* On the basis that turning the engine off was deliberate and not an emergency, I don't think the scene after that point is particularly unrealistic. They appear to be making a straight in approach and land on an airfield - it's entirely plausible the Captain made a deliberate decision to land with no engine from the very start.
*Example: