The TOCs and their staff come across with the view that their job is to move trains and it is not their job to move people. If people happen to be on board that is their choice.
This is why some rail staff don't have the best customer service skills. They don't see customer service as their job.
I don't think that this has been my experience as either a passenger or a customer.
I can give examples where staff have gone out of their way to create tickets for passengers who are too young to be customers and staff who have been tasked on a daily basis to collect a blind customer from a train and navigate then around the station - in doing so chatting to them as they do so as it was often the same few staff. I've seen staff allow customers to use out of service rolling stock to get further on without having to wait for the next service (which they probably shouldn't do).
I'm sure there's more, and I'm sure that if we were to ask others they'd be able to come up with other examples.
Of course I'm sure that I could think of examples where the staff haven't been quite to the standard that they should have, but that's the case in all customer facing roles (and indeed could be down to then having an off day due to external circumstances - for example as a regular user of trains I rarely need to ask questions, however when I do it's likely to be at a time of disruption and so probably they've been asked the same question 100 times in the last half an hour and so probably their smile has slipped a little).
There was a thread many years ago where this idea was discussed
I believe the outcome was that just going tall would likely be cheaper than the European approach, but would probably still be ruinously expensive.
The railway is stuck with its tiny loading gauge, probably forever.
Forever is a significantly long time, over time all bridges will need replacing, most could be rebuilt (maybe with a bit of track lowering) to facilitate a over height gauge. Therefore given enough time it's possible without being ruinously expensive (note I'm not suggesting that it would be cheap).
Having said that, it's probably best to do this for all new lines (especially high speed lines) regardless, as then you've got a clear route you can deliver the double decked services rather than there being 4 troublesome bridges on an existing line which blocks the use of double height trains.