You can sign up to the Cloud to use free Cloud Wi-Fi hot spots in London. Cloud Wi-Fi is available in Pizza Express, Caffe Nero, J D Wetherspoons, Pret-a-Manger, Wagamama and 79 London Overground stations. Some venues do limit the amount of time you can use their hotspot and a small percentage charge. If there is a fee for using the Wi-Fi you will be informed before you connect.
You can use The Cloud hotspot finder to see hotspots near you.