A few controlled airports in Germany have a delivery frequency which is used to create those ad-hoc flight plans for leaving the control zone. They also provide the startup clearance for IFR flights.
The rules are simple. Any flight in controlled airspace requires (a) a flight plan and (b) clearance.
How it is done in practice is quite different. The most restrictive version is, for example, in the Netherlands
– full flight plan via your favourite filing mechanism
– clearance via ground/tower, whichever it appropriate for the airfield
Next less restrictive is what is practiced at many places in the UK
– [abbreviated] flight plan per telephone (“Booking Out”)
– clearance via ground/tower, whichever it appropriate for the airfield
Then there is what is practiced at many towered airfields in Class G in the UK, and in Germany, and in the US.
– [abbreviated] flight plan per radio on initial contact frequency (Delivery, Ground, Tower, as appropriate)
– clearance received as normal
As part of the plan and/or clearance you can request and/or be given a route.
Other variations are how convoluted the clearance is. UK can be extremely formal (“Your clearance is available, are you ready to copy?” etc etc), in many places it is as simple as “right turn after take-off, depart via ”
They also provide the startup clearance for IFR flights.
Not for VFR flights?
For IFR, the EOBT is brakes-off, not engine start.
For VFR, the EOBT is getting airborne (IIRC).
Not for VFR flights?
VFR flights do not need a startup clearance, that is an IFR thing (“request startup” → “startup approved”). In real life (at least in Germany) one starts the engine in a SEP without asking, ATC have no problem with this, knowing that SEP batteries are not strong.
I can’t answer the original queston about formal rules.
I can only relay my experience that when we operated from Dublin Airport, it was a requirement to contact “Clearance delivery” before moving on to “Ground”. Clearance delivery did little more an confirm they had my flight plan, give me my squwak and a departure route which would invariably be later changed by the tower.
When I have departed VFR at a controlled airports, in all cases (from memory), I have had to complete a ‘booking out’ form which goes to ATC. After I have received runway line up clearance, ATC then say Nxxxx cleared for takeoff runway xx, left turn, not above altitude xxx feet – or something along those lines. This typically happens where the departure airport is not the controlling authority for the Class D and above airspace above it’s zone, so no clearance to enter CAS is given – and unlikely in my scenario as if I did file a flight plan, it would unlikely to include entrance to CAS.
VFR flights do not need a startup clearance, that is an IFR thing (“request startup” ? “startup approved”). In real life (at least in Germany) one starts the engine in a SEP without asking, ATC have no problem with this, knowing that SEP batteries are not strong.
Not correct for the UK. Bristol Filton required SEP’s to request start up clearance, as do Norwich from what I recall.
Having read all the interesting posts here, and having originally reviewed the lettered airspace class definitions, it seems to me that the only clearance you should need to enter Class D or lower from the ground for a VFR flight is to be told on the radio ‘cleared for take off’. That’s the clearance, ATC is not required to maintain separation between you and any other flight, only to provide advisories, and since you are in VMC then See and Avoid applies to all traffic.
It sounds easier to me to trailer the thing out of there than depart some of the airports described. 
RIP Bristol Filton :-(
At EGNS (Isle of Man) which is in class D airspace, the procedure is to book out on the phone (abbreviated flight plan), then call tower for taxi instructions. Tower at some stage will ask if you’re “ready to copy” and will give you a clearance which is usually just the direction they want you to turn after departure and a squawk code. Occasionally they might give you an initial altitude restriction.
When I have departed VFR at a controlled airports, in all cases (from memory), I have had to complete a ‘booking out’ form which goes to ATC
Booking out is a UK only thing. I’ve never come across it anywhere else.
