Thanks to everyone who posted a reply to my question on 27 June. An interesting selection of answers!
Maybe I should explain I'm retired with a Senior Railcard, so I'm a 'leisure' traveller searching for travel bargains and booking on line. Most of my journeys include hotel nights and these are significantly cheaper if I book many months ahead. This seems to be the general trend these days, airlines being another example. Maybe I've just been lucky but booking way ahead has never caused me insurmountable problems, however the cost savings can be huge! So huge that you can consider visiting places that you might not normally be able to afford.
I understand there will always be engineering works on the railways but the core services seem to run year in, year out, with little change. So why not allow bookings a year ahead, with the provision that nothing is guaranteed till 12 weeks? This would seem reasonable if there were really big discounts to make up for the uncertainty, and a complete refund was always offered as an alternative to a forced change.
I think there's a bit of natural 'resistance to change', which is understandable in a system that's been around for many years. Long range reservations might be unusual but there doesn't seem to be any insurmountable reason why they couldn't work – given the will.
Thanks all. See you on a train.