Britbox is not currently available via a NowTV device (only a full blown Roku Player) and i have no idea if Britbox will eventually turn up on NowTV devices sometime in the mid or long term future.
Here's a link of the current playback devices that supports Britbox.
Unfortunately Britbox have not yet developed a app that will work on NowTV Box/Stick [Essentially a custom roku device] they may in future but it is not I believe on the roadmap i think amazon is there next device to develop for.
There's already a Britbox app for Amazon Fire devices though. I don't get why they can't take the US britbox app and use that? It wouldn't take much to adapt for a nowtv stick.
