How do you know that what you suggest is a delay is not built into the schedule? The time of departure is irrelevant. All that matters is the delay at your final ticketed destination. If that departure delay translated to an arrival delay of 35 minutes, it would not meet even the minimum delay for a Type 1 flight under EC 261/2004. I can't imagine that BA would voluntarily pay any compensation, let alone even a minimal customer service gesture.