The history being it was considered to be less workload for pilots (students especially) to (A) set 0447/0457 before takeoff then (B) Farnborough would issue a discrete code beginning 044 or 045 to the pilot when he/she calls thus requiring only one digit to be changed and avoid the risk of the pilot inadvertantly dialling through 7500, 7600 or 7700; simple eh?
And yes, Farnborough would prefer the request to route south from Blackbushe be made on RTF, thereby they can treat each individual aircraft on a dynamic basis eg 'route east side' (to avoid 24 departures) or 'route west side' to avoid 24 arrivals or 'through the overhead' (no arrivals or departures); of course it will be reversed if runway 06 is in use!