Port of London Authority River Byelaws 1978 (as amended) Part 5 requires a bundle of straw "large enough to be conspicious" [sic] to be hung by day from any bridge reduced in span or height. At night they just specify a white light, but that's much less fun. The farm food has also been hanging from other bridges recently (notably Hammersmith and Blackfriars) but they're not quite as photogenic.
Work's going on until November so you've got plenty to time to check it out, and perhaps also note down the dates of overnight bridge closures. Image by Nick Holdstock from the Londonist Flickr pool