In general, when booking your flight to London, March is the cheapest month on average.
From most locations, booking your flights to London around 23 days prior to your departure date finds the best deals.
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 morning departure times.