Pretty certain this is due to the CAA not allowing it. I guess the logic goes something like this:

1. No headphones should be in use during take off or landing so that cabin crew evacuation instructions (which may not be broadcast over the tannoy system) can be heard.

2. No IFE allowed until after takeoff in order to avoid people being tempted to wear headphones.

The authorities in Dubai and Australia obviously take a different view. On recent CX flights I note that the system was not switched on until we had reached cruising altitiude.

For another example of this kind of thing, note the continuing use of plastic knives on board flights to and from Australia, even though most other countries have relented on this.