Got G-TTOE on one of my recent flights and got to try the WiFi. It was short segment from London to Hamburg, so around 50 minutes in the air. Prices were £1.99 for messaging or £6.99 for streaming - easy to pay via my account (I was already registered). Speed was 620kbps on the streaming package - enough to watch YouTube, but not enough for Netflix.
On my two recent BA11s (London - Singapore, so 12h in the air) internet was priced at £14.99 for full flight messaging or £24.99 streaming. On the first flight internet got reset a number of times requiring me to pay again. I've asked for refund of additional charges and got refunded all of them. On the second flight internet stopped working a couple of hours into the flight. Speed was decent though: 6mbps on the steaming packagepackage.