The Grand Union does not have anything to do with the Lee and Stort.
Several rivers pop in and out of the GU..when those rivers do go into flood, several small sections are almost impossible to do going upstream...however, this never lasts long as the rivers have small catchment areas and drain well.
On the East side of London, the River Lee is for the most kept well separated from the Lee Navigation...and below Tottenham Lock is so well controlled that only exceptional rainfall gives it some flow and a few inches of raise....further up past Stanstead Abbotts there are again a few feisty bits and low bridges.
The Stort is more like a proper river and rises quite a bit making at least 3 bridges impossible and one lock inaccessible.