Do you have access to WiFi anywhere else (work etc)? You could download what you wanted to onto your phone before you get home and watch it data free that way.
Alternatively, I know you can get a sim only unlimited data plan for £20 a month with giffgaff. The first 9.5gb is normal speed but they do slow down the speed after that’s used up. But if you go on a price comparison site like uSwitch you can filter the best sim-only deals with unlimited data to find a better version. Just make sure you read the download speeds/limits in case there is any hidden details!