JAG,
It's nice that someone actually reads their plates... the number of pilots that don't say anything and don't make the altitude is a worry. All the airway SID's from 07 should have that warning.
All airways departures from Northolt are transferred by Tower direct to the Outbound TC controller, but the bit in between 2400 feet and the runway is airspace that is controlled by the Northolt Radar controllers. So to reduce pilot workload of having to keep swapping frequencies, the departures controller will procedurally release aircraft.
So if you can't make 3000' by the London Control Zone Boundary (thus remaining within the protection of CAS), the departures controller will (as part of the release approval) instruct tower to keep you on their frequency until passing 2400 feet (so you’re back inside CAS), whilst radar monitoring your flight.
This release approval will be given subject to the departure controller ensuring that all aircraft outside CAS will be at least 5nm away from your flight. So effectively you will be receiving a Deconfliction Service until you’re established inside CAS.
I hope that this makes sense.
Regards
MM