In general when looking for the cheapest flight to London, try to book your flight during the month of September for the best deal.
Most flyers found that booking flight tickets 60 days in advance of their planned departure date is, on average, the best time to get cheap flights to London.
On average, the cheapest flights to London are found when departing on a Wednesday. Try to avoid departing on a Saturday, as this is usually the most expensive day.
On average you can find the best deals on plane tickets to London when looking for flights with noon departure times.