So Just got a NOW TV subscription and went to cast to my TV, didn’t work, tried the Home Hub didn’t work either. Did some googling no help, contacted Now TV support, even less help so back to Google! Came across: https://community.nowtv.com/t5/Set-top-boxes-Smart-TVs/Chromecast-Loading-Spinning-Circles/m-p/530674/highlight/false#M10500
Basically enabling the Google integration on HomeAssistant breaks the ability to cast from NOW TV. This is the only affected source myself or anyone else on the above thread has found.
So the question is what is Home Assistant doing that stops the NOW TV cast feature working?
Even if we do not find a solution I hope this helps someone out that had the same issue as myself!