When do you have to call departing tower if you landed and don't have a flight plan?

True

You have received a clearance, for instance to leave controlled airspace, and that clearance ends with some kind of “report” when this is done. This is normal controlled airspace logic. Any sort of confusion or “back and forth” stems from the fact that this logic is masked/hidden in the normal radio practice. It’s good airmanship to prevent/minimize confusion. It’s not the ATC that is responsible for initializing this report, even if they often will do it as part of the normal radio practice. It could also in many cases be a VFR reporting point, in which the pilot reports of course.

As mentioned this isn’t the US ATC practice for leaving an airport although if it is warranted and if they have time, Tower may approve an early frequency change while you are still within their Class D instead of watching you exit without further back and forth. Also, it is rare to be asked to use a reporting point on a VFR departure, these are used mostly to sequence inbound traffic. This works fine to minimize clutter on tower frequency, on which there is often almost non-stop communication. In general Tower can see you on radar and when you’ve left their Class D they no longer have any interest in you, you’re out of the nest at that point and on your way in the big wide world If you come into potential conflict with arriving traffic once outside of the Class D, Tower will advise the arriving traffic without assuming you are still on frequency. Meanwhile the departing pilot is looking and thinking forward, not backwards.

This practice obviously evolved over time, in dealing with very large numbers of movements simultaneously on a Tower frequency that is as a result very crowded.

Silvaire wrote:

Also, it is rare to be asked to use a reporting point on a VFR departure, these are used mostly to sequence inbound traffic

Certainly not around here.

Anyway, this is all in SERA.4020 b, and obviously there was a misunderstanding of what the tower actually meant when they explained this.

Mostly for a VFR departure from a towered Class D airport in the US you request and are cleared by Tower for a left or right crosswind, left or right downwind or straight out departure without further details. You then do it, and may have no further communication with the tower (as discussed) prior to leaving the Class D for that airport. Sometimes, on the ATIS at some airports in my experience you may be asked to state “direction of flight” to Ground on the initial callup for taxi clearance. Ground will then send you to an appropriate runway, and Tower will issue the takeoff clearance with something like “northbound departure approved”. In the absence of further details, how you execute that is up to you, although if you want an early turnout I think its wise to clarify at that point. Tower will watch you visually and on radar and if there is anything useful to say before you clear Class D, they’ll say it. If not, they won’t. Tower does not give instructions on a frequency to call after leaving the Class D because the baseline assumption is that VFR traffic won’t be calling anybody, and anyway that’s the pilot’s job to determine. They also do not need to provide altimeter settings etc to VFR traffic because the pilot gets that from ATIS before making the initial outbound or inbound radio call.

This is how Tower deals with many more aircraft simultaneously than would be expected in e.g. Europe.

The answer to the simple original Q is NO, in the UK or anywhere else I have been.