This would not use Roku's billing, the channel would be free. All money would be handled by Betable UK with it's UK gambling license. Placing bets etc. would be done using REST API calls over HTTPS.
The reason I ask is that places like the iTunes store have been getting more liberal in the last few years regarding gambling. Right now the Roku UK market probably isn't big enough to support serious gambling development efforts, but I see real money gambling as a big growth area that is a good fit for big screen TV viewing and it fits well into what I know of the existing Roku demographics.
