The best time to trade FX is anytime other than at the weekends... I know traders as well as myself who have been up at rediculous hours to trade the start of a potentially good move.
However, if you are asking what is a good tiem to trade gbp/usd then the uk and us sessions are the more active. This doesnt mean to say that the asian session is a waste of time personally i'm usually in bed and therefore dont trade it often.