However there are reports of TOCs that offer Weekend First telling passengers they must purchase the Weekend First supplement to travel in 1st class accommodation at weekends.Yes in the days before Avantix if no first class fare was shown and first class accommodation was offered on the train you could simply add 50% to the standard class fare and that would be the first class price.
It helped to keep the size of the fares manual down as you did not need to include first class fares for those routes where only 1 or 2 trains a day offered first class.
However with the current system you are no longer allowed to create fares. Its either there or it isn’t. I think it would confuse the ORCATS because it would not know how to divide up the revenue because no one has told it.
What usually happens in these situations is that people get to know about it. They sit in first class and when the guard comes around will smile nicely and say they tried to buy a first class ticket but one was not available. Guard will then complain to their manager about people sitting in first without a first class ticket and the next fares round a first class fare will be loaded into the system.