Just discovered that the GWR main line to Paddington now has free wifi. It used to be limited to just some GWR lines. Train moderately full on a Saturday, Speedtest showing about 0.6 Mbps download, quite a decent ping too. No streaming, not even audio, possible. Stutters occasionally but then recovers. But perfectly OK for posting in places like HUKD and email.
So this is just a heads up so you can know to take an iPad/tablet on the train and no need to faff with a phone hotspot.