It's highly unlikely that giffgaff will do something similar in the short term.
If and when it is introduced, (which I believe it probably will be at some point), expect it to be a chargeable extra, like Vodafone do it because that's the only way it's really sustainable, and it'll likely only be available via higher cost packages.
Three, when they did similar absolutely wrecked their network performance by making it available to people paying as little as ~£10/mo which meant they had huge numbers using huge amounts of data and paying very little money for the privilege and trust me, it was noticeable. Locally before "go binge" launched I'd get 40-60Mbps, by the time I left I was struggling to get 5Mbps some days.
giffgaff would similarly grind to a halt if they made it free and available to pretty much everyone, which would be commercial suicide now they've finally sorted their network issues after many years of waiting.
The Voxi package you mentioned is good value at the price point but be aware of the caveats, (the terms and conditions), because there are many ways in which you can come unstuck unless you read them and stay within them.
They aim it at under 30s however there's no age restrictions.
You just have to be alive and kicking.
@williamwalker I tried to order a sim and when I clicked on the box that said I'm over 30 they refused to send it:
"Nothing personal, but it's designed for people under 30."