The River Thames, once past Teddington Lock is a tidal river. In other words there are moments when the level of the water rises (high tide) and moments when the water levels drop (low tide). The River Thames, throughout most of London is not so much a river, but n inlet of the North Sea, subject to the same whims and ways as the larger body of water to the east of England.
So, it should come as no surprise that there is a beach on the River Thames. At low tides, you will see finely washed sand throughout the whole length of the river from Teddington to Southend (culminating with the beach at Southend Pier).
I am not going to advocate stripping off and going for a swim in the river and a bit of sunbathing along the South Bank. The tide rises fast, and you can get stuck very quickly on the foreshore. But just to show I am not lying, here is a picture of the beach on the Thames, London's very own seaside.