Circuitous routes out of Biggin Hill
I have idly noticed every now and then that certain aircraft (usually powerful single and twin props) heading generally West from Biggin Hill do a long-ish departure to the East and South steadily climbing up above traffic in and around London. I can understand why this might be necessary for the more feeble prop planes ;-) but for the more powerful and capable props it seems a waste of fuel and time. Obviously it is deemed necessary by ATC, so would someone be kind enough to explain why please ?