However, that is irrelevant with Farnborough clutch departures since one would reasonably expect that a flight being held outside with an EOC time 30 mnutes away would be working Farnborough who can and will provide a DS rather than being limited to a BS with TC.
Again, not how it works. May I respectfully suggest you visit either TC or Farnborough ATC? Both paties (pilot and ATC) will benefit from such visits but it will show you how LF clutch departures are handled.
LF calls TC and speaks to the coordinator for departure approval (not departure release, but
approval, as release implies a IFR service from TC from th eoutset and management deem that to be too dodgy). No OCT is given and no indication of when you will be given joining clearance is passed - approval means merely that looking at current workload, the controller can handle an extra flight (taking into account the joining fix, aircraft type etc for complexity issues),
at the time of the phone call and in the immediate future.
The aircraft gets airborne and subject to Farnborough traffic is given a climb which will keep it clear of CAS.
90% of the time the aircraft will be instructed to remain clear and handed over to TC for joining clearance. Fanborough will do this when the aircraft is clear of immediate confliction.
About 10% of the time, if there are too many conflictors, LF will call for higher to get you above traffic, possibly into CAS. Oftne this will just be 5000' under a Heathrow or Gatwick departue, so you could pop into then back out of CAS.
TC will give you further clearance as and when it can, sometimes you will be waiting a good 5 or 1o minutes especially towards the Hazel/GWC area as this is quite a busy area for TC/LF/HI.
LF departues via CPT tend to work better as there is more flexibility with airspace to the south of CPT, if you were to be put on a westerly heading (for example) for join.
The real problem is for training flights from Oxford to Bournemouth (for example). They tend to be poor performers and getting clearance to join at CPT at even FL70 can be tricky.
The bottom line is, Farnborough will not hold onto you under a DS until TC can give you joining clearance - they will hold onto you until they deem you to be free of confliction... what happens betwen then and joining clearacne from TC is the sticky bit, as far as you, the pilot are concerned.
Needless to say, if you are in an aircraft with decent performance, you will get clearance sooner, as it is easier to integrate you with the other traffic.