All published Standard Instrument Departures from Biggin Hill route towards Detling VOR initially. DET is 21 nms east of Biggin Hill. From there, published routes continue mostly to the north-east, east and south-east; the only exception is the route towards Brookman’s Park, Henton and then Compton.
ATC May of course offer earlier westerly routings, once under radar control, without the need to fly over DET.
The initial route keeps traffic clear of London City, Gatwick and Heathrow and this is probably the reason for the 21nm easterly route before any turns are published.