The AIC that Jim mentions applies during the Olympic festivities. If you are talking about routine operations it probably comes from the fact that a delay of up to 20 minutes will not be advised to pilots. It's not my area and I'm sure someone working the LTMA can give you chapter and verse on the exact wording - but I think it goes something like if the aircraft will leave the hold less than 20 minutes after arriving it will be given an EAT of no delay.