> The UK has a hybrid model which is also in use in countries like Germany, which means that seat reservations can be made on many trains but aren’t compulsory.

> This optional seat reservation system is so much better as it means that trains can technically never sell out and you can always hop on board of a certain service.

The UK system is starting to move away from this "get up and go" model, unfortunately.

A number of train operators are now setting the "Reservations are mandatory" flag on their services in the timetable data, which means that online journey planners cannot sell walk-up tickets if there is not seat reservation availability (or indeed, if reservations have not yet opened which is increasingly the case for west-coast services until very close to the travel date), even if it would be possible to buy that same ticket from a machine at the station or in-person at a ticket office and it would be perfectly valid for said service.

It ends up with really poor UX.