Hello @chrisdgrigg and welcome to the community.
unfortunately only the BT TV Pro Box offers the ITVX App at this time, if you have a Pro Box this app should already be on the Pro Box.
If it’s not on the Pro Box, please follow this link for more guidance.
The app on BT TV set top boxes is listed as ITV .
Depending upon what box you have if it is a (Sagemcom) BT TV Box Pro or Mini then that ITV app provides (reduced) ITV X features whereas the (Humax) BT TV boxes provides ITV Hub.
For full features and higher quality ITV X content you will currently need to use the app on a different device.