Three are offering for free on all 4GB Advanced plans and above 'Go Binge'. There are certain partners they have whereby if you use their service it doesn't come out of your data allowance. Partners at launch include....
So use any of the above services and the data is infinite as they call it.
I put £12 in as the price as it was the cheapest plan on their site that it was listed on. I'm assuming £12 is the cheapest 4GB plan they offer.