By telephone, 340 Lindens for a £1 Call
By SMS text message, 300 Lindens for a £1 message
And by using the terminal in-world at the DX HQ in London City...
410 Linden Dollars for £1
Visit DX Exchange at their UK HQ in London City and get great rates when buying Linden Dollars http://maps.secondlife.com/secondlife/London%20City/193/141/24