Just flew ZBKK, the WiFi was not activated until half way into the flight, the pop up showed on the IFE screen saying we could now connect to WiFi with no prior announcement. On decent they announced it would stop working shortly. This was on LHR-BOS, could it be that there is no coverage over the eastern Atlantic once off the shore of Ireland? But the climb to FL400 took around 25mins, so WiFi should have been available just a few minutes after take-off.