The link below has all the detail but basically, the mayor has the tender out soon. No more having to jump between station Undergound Virgin media WiFi for partial connectivity! Yay! https://www.standard.co.uk/news/london/tube-to-have-full-4g-coverage-within-two-years-pledges-mayor-a3608346.html
Link to article: http://www.bbc.co.uk/news/technology-40738947 Whats more interesting is further down in the article. The UK government actually looks like they are selling spectrum early instead of turning up to the party late like they did with 4G (there were some legal battles as the mobile operators had paid so much for 3G).