The current London Underground setup doesn't require a mobile SIM. It is also available to VM broadband customers.
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Choose Virgin Media Broadband customer, or Virgin Mobile customer and enter your username and password.
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Once you've activated Virgin Media WiFi (either as a customer or with a WiFi Pass), your device will automatically pick up our WiFi signal when you're in a WiFi enabled station.
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The Wi-Fi Buddy app isn't just for VM mobile. Eg it works on my Hudl2 tablet. With it at home, I can see another VM broadband customer, a Sky customer and a BT WiFI with FON setup all within range.