OK, so I just ran the numbers....
That section of Bowness is approximately 200,000 square metres. Lets assume for simplicity it is filled evenly to a depth of 1 metre. The full flow of the Bow at 2000m³/s. So Bowness could hold 100 seconds worth of 100 year flood water. Blocking that will have very little affect downstream.
How much money would you spend on a reservoir that could hold 100 seconds worth of water to protect a park?