Are people in the UK that lazy they can't go on the train companies website and book a ticket which comes with a seat reservation? Can't use the internet? call up the booking office or travel to a local station the day or 2 prior to book in person. Some trains in Queensland you have to book weeks in advance just to get a ticket. Eg Brisbane to Cairns trains. But people accept this and book well in advance.
Why should you have to?
"Those who don't book in advance will learn the hard way" - you mean those on the most expensive, revenue generating tickets?
Let's say I need flexibility due to a meeting so book an Anytime Single from King's Cross to Edinburgh, £193.90. I book a seat reservation for a train that is hopefully suitable but the meeting starts late - there are no seats left on later trains that day but I still need to travel. Your answer to that would be "Train full! Bad luck book on another train!"?
What about if your train was cancelled due to a fault, but all the later trains were fully booked. What do you do then? Push all the passengers onto a train the next day?
The fact is that the majority of UK rail, even long distance, is somewhat turn-up-and-go. The distances involved are nothing like Australia. It destroys the advantage of UK rail that if you buy a flexible ticket you have the freedom to travel whenever you want. The railway should be encouraging people to use it, not turn them away through a complete lack of flexibility.
Not to mention this doesn't solve people sitting in the wrong seats.