The first part of each SID, up to 4000 feet, is defined as the Noise Preferential Route (NPR).
NPRs, as you rightly say, have remained unchanged for many years and neither Heathrow nor NATS has the power to change them, only the Government.
The historical reasons for their routes are lost in the mists of time, but they were likely defined to take account of issues like populated areas, etc, as they were in the 1960s. A further reason why the BPK SID doesn't mirror the tight turn of the DET SID could be potential conflict with Northolt traffic.
You would probably get a more definitive response if you did ask in the ATC forum - they are quite friendly over there.
