Why is this such a common problem with the shop (at least 1 in 5 journeys ime) but not with guards selling tickets and upgrades?
I don’t know, but I would hazard a guess that the revenue kit is set up to push through a transaction regardless whilst catering, being an entirely optional purchase, simply isn’t?
Data signal on trains in the UK is, frankly, rubbish. I’m sure anyone who traverses certain routes regularly will know that there is no signal between here and here, or just after leaving there, etc. On board wifi just appears to mimic this, with a slightly better connection in some places but nothing dramatically different, IMHO. I can immediately think of one significant railway station in a sizeable city where you struggle to get any data or wifi signal sitting on a stationary train in the platforms, and I’m sure there are others. Maybe the DfT aren’t being that unreasonable wanting to do away with it, given that most of us now carry a 4G/5G data connection around in our pockets and the wifi they install and run at significant cost seems to offer little extra capability much of the time.
As for catering, the answer here is surely just for the affected TOCs to identify the areas where connectivity is affected - this should be broadly the same every time - and work around it; i.e. advise customers that between A & B no card payments are possible owing to the geography, etc. It isn’t ideal, but it’s better than it seemingly being totally random when in reality it probably isn’t!