It's a similar situ with the FireTV devices mate.. The Chromecast most likely has a different GoogleTV version.What's odd about all this though mate is that BBC ITV and 5 are on the google TV Chromecast so I am not entirely sure its the platform which is the problem
For example, the FireTV Cube used to have the full-blown iPlayer [4K/UHD/HLG]
A while back the HLG part of the app broke and it can now only play up to UHD level (without the HLG)
The new FireTV Firestick 4K Max however can play the full spectrum of iPlayer [4k/UHD/HLG]
The only difference between the Cube and the Max is the FireOS version of the Max is a slightly higher version, the actual iPlayer app is exactly the same ver across both.
It's all about the underlying OS that makes it just so messy when it comes to iPlayer compatibility.