By the South Bank there is a tiny strip of beach. If you look down you will see something the picture above - mud, rocks and rubbish. Of the rubbish the majority is non-bio-degradable plastic, the small bits that are getting smaller to the point they become ubiquitous.
There are rubbish collecting rafts, like the one below. But they only collect the larger pieces - such as in this case where the one opposite the Tate Modern contained six ring lifebuoys.
Its hard to see how the smaller bits of plastic can be removed. They will remain in and by the river Thames for decades if not centuries.