well its not an easy question to answer. I havent flown out of Biggin for years but if you carry on in a straight line you are flying straight at aircraft approaching Gatwick. You are totally reliant on ATC at Biggin coordinating immediately with Gatwick and the message getting through quickly enough. You will probably be too low to get an RA and you are flying towards rising ground. The SID, although turning, is over level ground and keeps you clear of traffic. In my aircraft I have data for the degredation in a turn at 15 degrees of bank so if I can make the gradient I am going to stick on the SID. In the case of Biggin I would probably be limited by the length of the runway but I haven’t actually done a perf for it. I just think its an interesting problem - my previous company mandated to pilots that they must use an escape if it was published. I tried pointing out that if I was departing 36L in Beijing and at 500 feet I commenced a turn straight across the climb outs of the other two runways I would be supremely unpopular but they werent having any of it..