London Gatwick airport, south of the capital, has lots of Wi-Fi options. Coverage in the south terminal at the airport is provided by BT Openzone and The Cloud. Then you can see T-Mobile from some locations too (e.g. in Starbucks up stairs in The Village shopping mall)
The picture, by the way, is the Gatwick Express train which takes you directly from the south terminal at Gatwick to London Victoria train station in 30 minutes.