Been a Curve user since the very first Beta in 2016 and a Metal cardholder now - registered simply to upvote this post as wanted to ask exactly the same.
I buy train tickets every week and cashback on Virgin purchases has been the best single Curve use case I have. I don’t even travel on Virgin much, but GWR’s website/app is hopeless and so I’ve been able to book GWR tickets in Virgin’s app in twice the time and get cash back too.
I hope not, Avanti’s website is as bad as most of the others sharing the same outsourced payment experience (and don’t think they have an app). Virgin was unique because not only did Curve give cash back, but their app and website were the best in the business for train ticket booking.
If it has to be another I would go with GWR, as bad as it is, most of my tickets are for them.