They don't need to publicise it.
For example, the advance fares for London - Caerphilly are:
http://brfares.com/#fares?orig=1072&dest=CPH&rlc=
£12.50
£16.50
£20
£24
£28
£31
£35
£41.50
£56.50
£72.50
£88
The SOS fare is £103.50
On some peak trains they won't sell any cheap Advance fares, ever.
On some off-peak trains they might sell £12.50 Advances in say February, but then due to known higher demand in Spring/Summer cease to sell those fares in say May.
Basically while all those fares exist, they might not release any £12.50 fares at all for a given week, and won't release any such fares for a given train either.
Obviously if you are accustomed to buying advances at a certain low price, then it is indeed a fares rise. But it's not one they need to publicise, since all they have done is reduce the number of seats sold at the given price to zero.