VISA for 6 months time

Hi,
I am a UK citizen planning to visit China in June or July at the end of 6 months of travelling. I am led to believe I cannot apply from the Embassy in london before I depart on my 6 months travelling as it is valid for 3 months from the day of issue. Is it possible to apply in the embassy in london for one starting in 5 months time (e.g. May) for it to be valid for 3 months after that?

Thanks