Thanks for the comprehensive reply - I didn't realise that the latest video and gaming ran so fast and had such large files that need to be updated regularly!
For reference, when I was working on it, BT Sport contribution, (ie from Outside Broadcast site to Stratford studios), for 4K ran at around 100 and 140Mbps - that is the high quality feed before it was processed for distribution to customers at a lower bit rate.
Probably makes sense for Openreach to install the 2.5GB ONT modem as it saves them having to send an engineer out when an upgrade above 900 mbps is requested. Looks like Cornwall is one of the test areas