Only a few train fares, such as standard and open returns, are regulated by the government; the special deals, etc, are down to individual train companies who are free to produce / use their own schemes. When First took the enlarged Greater Western Franchise, they rapidly withdrew the cheapest buy-on-the-day, off-peak ticket - the supersaver - which was a return ticket available on most days (but not the very busiest), and not on rush-hour or near-rush-hour trains.
The new single tickets are only for sale for travel by a specified train, and must be bought by 18:00 on the day before travel. They're also only sold in limited quantities, so once a train's allocation has been sold, the price rises - you have "A", "B" and "C" tickets in both standard and first classes. Sometimes, (I've found it), it's now cheaper to go first class than standard.
First's marketing tells you that their simplified new pricing (ha,ha) is designed to let you get a low price fare if you're making a single journey, and so that you don't have to pay for an off-peak, planned journey at full rates just because of the time / date of your return. These are good points. What they don't tell you is that the withdrawl of the supersaver has allowed them to put up the price for pay-on-the-day customers to help them make more profits (their annual report boasts of 10% dividend increases for shareholders every year, and gives raised prices as one of the ways they're doing it) and helps towards the 1.1 billion pounds that they're paying the government, over 10 years, for their franchise - in effect that money is a tax on train travel; it's far more than the Department for Transport expected to have bid, and the excess is being used to fund other DfT expensiture such as on road programs.
How can you get a good fare?
1. Yes, do book well ahead and choose a different train time
2. Look at splitting the journey. Chippenham to Taunton last weekend was 33% cheaper if you booked Chippenham - Bristol and Bristol - Taunton. Melksham (our home town) to London - much cheaper if to book Melksham - Didcot and Didcot - London. But you MUST use a service that calls at the intermediate station.
3. Check out other operators. For London - Bristol and London - Exeter, South West trains also provide services (from Waterloo). The Bristol one is only a couple of trains a day and is slower but a tiny fraction of the cost. And sometimes Waterloo is a much more convenient London point than Paddington.
For London to Cornwall, try a "mix and match" or 2 and 3, rebooked at Exeter. Remember that all ticket offices can issue any of the tickets, so you don't have to physically rebook.
From our own (local) viewpoint, the timetable that's being provided by the new franchise operator from December will be a disaster - just 2 trains a day each way where previously there were 5, with all the cheap-time trains withdrawn. Our only "outbound" train in the morning will be at 07:17. Supersaver was 38 quid. Gone. Save is 45 quid. That also goes in December because the 07:17 is classed as peak. Fare rises to over 100 quid. And then goes up again in January, I expect, with the annual price 'review'. Via Salisbury to Waterloo, it's about 60 quid.
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P.S. I should stress that First is a commercial company and it's actions appear in this instance to be within their contract terms - their policy and its efffects are all above board under the system as it's set up. And they're the natural actions of an organisation who's "raision d'etre" is to make money for shareholders. It's the system itself which is doing a dis-service to the passengers.