As a rule, Volo TV facilitites are found in Coach D of FGW HST sets. They feature, among other things, maps, news and TV. On Monday evening I was quite content to lie back along the Golden Valley watching the speed and altitude change real-time. And having had the facility through the Box Tunnel a few times, I can confirm that the system does appear to work in tunnel. Whether it works under the Severn is another matter. I'm not sure how many times I have had the facility now, probably about five, but I always try for Coach D if not reserved or First Class now.
As for several other criticisms, watching TV at point blank range is a big plus for the average young traveller, especially given there is a comedy selection. The whole system is I believe £3.95 per journey (whether short or long) for programmes, and free for information (including map and speed data). There is support available, either over the phone or by e-mail. Having said that, the latter isn't that useful without wi-fi...