Even in 2012
there were plenty of buildings in central London that exceeded the c.40m height of the SAMPSON and lower S1850M antennas which would have interferred with the radar picture. Given the point of T45 is to identify, track and interdict aerial threats at very long range, it would have been daft to stick one in confined riverine waters, although the plan was to have her at Sheerness rather than in London. One assumes terrorist threats would have arisen close to the venues with short range weapons which could have been easily hidden. The stationing of Typhoons at Northolt, the Rapier and Starstreak batteries, and Giraffe radar was a more sensible solution. Thre was also availablity of E-3 coverage in those days.
But then I suspect it was all to be seen to be doing something rather than a reaction to intelligence received.