This does work in many other European countries, but there simply isn't enough rail capacity for this in the UK. Turn up to any mainline station during peak ho

This does work in many other European countries, but there simply isn't enough rail capacity for this in the UK. Turn up to any mainline station during peak hours and you can observe this for yourself (also non-peak hours in many cases). Trains which require reservation have scarcely any standing room. Other countries would be quick to complain if you don't get the seat that you paid for.