Travelling from most locations, it is best to book your flight to London during the month of October for the best deal, although there are other times of the year which offer great deals as well.
On average, from most locations, when planning your trip to London, try to book your flight in the region of 60 days in advance for the cheapest prices.
In general, booking your departure to London for a Tuesday represents the best deal on airfare. Try to avoid a Friday departure to stay away from the highest prices.
On average you can find the best deals on plane tickets to London when looking for flights with evening departure times.