You can use wi-fi or mobile data on the Tube at no extra cost by connecting to your provider's service. The current wi-fi network providers on the Tube include EE, Vodafone, Three, O2 (Wifi Extra), and The Cloud (on London Overground). While 4G and 5G is available from Three, EE, Vodafone, and Virgin Media O2.
The VirginMedia SSID has vanished from the London Underground network, some time during the day yesterday 30th March 2023. After doing some digging I found an article in an ISP news website about the HotSpot service ending later this year.
When travelling by train in the UK at the moment you have access to wifi onboard around 90% of train services. Here is the most up to date information provided by each train company regarding the wifi service they offer.
London offers free Wi-Fi in many areas throughout the city. Major tourist spots such as Trafalgar Square, Hyde Park, and even the London Eye all offer free Wi-Fi. There are also other Wi-Fi hotspots available, often located in cafes, restaurants, and public libraries.
After you've connected to O2 Wifi, you'll be able to use Wifi on the Underground in 24 hours. While anyone can use O2 Wifi, you must be an O2 mobile customer to use Wifi on the Underground. If you registered for O2 Wifi using a number from another network, then you won't be able to use Wifi on the Underground from O2.