Slightly off topic, but germaine to the general confusion about mobile ticketing: NRE describes an m-ticket thus:
An m-ticket is a train ticket held on your mobile device in a rail travel app. Just download one of the free apps and purchase a ticket online for participating stations.
Very poor wording, given you do not
just have to download the app and purchase a ticket, but also activate it!
https://www.nationalrail.co.uk/times_fares/ticket_types/191371.aspx
EDIT: Yes, if you read all the way to the end of the page, it does tell you:
If you have an M-ticket in a mobile app you will need to activate it before the barcode can be read
Which is still not a correct explanation of the situation, I think. It's not acceptable to only activate it when the RPI asks to scan the barcode, is it? I'd always assumed it's more like validating ('composting') paper tickets in France and some other European countries, where you're in big trouble if you don't validate it before boarding...