Nothing prevents you turning below 400 feet except tribal knowledge. There are plenty of departures that require a small track change very shortly after take off. I have flown that departure from Macau a few times and, as you discovered, you will have to be very light not to overshoot the required track if you don’t fly what’s on the plate. If you do it at Biggin Hill you will cause the turn back to the overhead to take you through the edge of Kenleys perimeter. Be wary of tribal knowledge chap.