Partner with UK mobile networks to offer free data without using your allowances viewing/using Twitch mobile apps.

Some of the UK mobile networks already offer this service with certain applications.

Voxi (a Vodafone sub brand) unlimited data for Snapchat, WhatsApp, Facebook, Twitter, Instagram, Viber, Pinterest and Facebook Messenger as standard. Any company can apply to get their app on their service ( https://www.voxi.co.uk/partners )

Three offer Go Binge which allows free data for Netflix, TVPlayer, Deezer and SoundCloud.

EE offer Apple music and the data to go with it free.

You would need to reach out to the other mobile networks but don't think it would be an issue for Twitch.

This would increase viewership via mobiles/tablets in the UK, you maybe able to replicate this around the world depending on the local mobile networks.