Please be aware that London has multiple airports and that these results are only for London Gatwick Airport.
At London the most flights depart around 07:00. (Hourly flights, based on 240 datapoints.)
From London it is on average the cheapest to fly to Istanbul and to Dubai. (Average prices, based on 90 datapoints.)
From London most of the flights depart to Amsterdam, second best connected from London is Rome. (Daily flights, based on 59 datapoints.)