We are looking for the best way to reach Borough Market from Heathrow Airport. Thanks for any help
Piccadilly line to Green Park, then Jubilee line to London Bridge, then walk about 300 meters.
Citymapper is an app I've found to be reliably helpful. It shows many options from Heathrow to Borough Market during the day on a weekday with journey times ranging from 50-65 min. However, most of the options involve 2 changes of train. My preference would be the option with only one change: Piccadilly line from Heathrow to Green Park station then Jubilee line to London Bridge station. Journey time 65 min.
The locals might have better advice (which you should take!)
Sam said it much more concisely!
Green Park is notorious for its very long walks between different lines, but there isn't really a better way.
When changing at Green Park, don't follow the signs to the Jubilee Line. Ignore them. Go up the escalators to the booking hall, then follow the signs up there to go back down to the Jubilee Line.. Much easier and quicker than following the maze.