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The events of the last week... with the Jubilee River successfully carrying vast amounts of water safely downstream past Wraybury towards the sea... must surely show that the Jubilee River has been a resounding success, giving the environment agency more options to avoid serious flooding from a surge hitting London later in the week. The floodplain around Windsor and Maidenhead is the last major defence to save the capital from serious flooding, and we should be grateful that the scheme is working so well and that the threat to London has been contained.

Clearly the capacity of the upstream floodplains is relevant. In an extreme example, if a 100m wall were built across the Thames at Windsor, and a constant flow maintained in the Thames below, then London would have very high protection from the Thames flooding.
The worst scenario for the lower Thames (Windsor, Wraysbury, Staines) is for there to be a 'surge' coming down the Thames from Oxford/Abingdon at the same time as 1) the lower Thames flood plain already being saturated/flooded and 2) there being more recent rain creating surges along the Kennett, Lodden, Thame and 'The Cut'. There were elements of this in 1947: heavy rain on March 10th filling the upper Thames, then thawing snows and heavy rain on March 14th filling rivers in the lower Thames.
On this occasion (in 2007), the flooding might be compared to one of those champagne fountains. The Thames valley was filling up at the top where the very heavy rainfall was, but there were enough empty glasses at the bottom to avoid flooding. Windsor and Wraysbury were fortunate that, by the time the upper Thames 'surge' arrived, the Kennett, Lodden, Thame and 'The Cut' were no longer on flood warning, and the floodplain around Windsor and Maidenhead had not become saturated, either from floodwater or fresh rainfall.