[astrosi]

It depends on the airport that you are travelling from. London City has famously fast security, currently you can check-in upto 15 minutes before the scheduled time of departure for a flight to New York. [1]

[1] http://www.londoncityairport.com/travelandbooking/page/check...

[jacquesm]

When flying in from Amsterdam that 'famously fast security' takes more time than the actual flight.