I believe it's against the rules of impartial retailing. Try contacting ATOC and Passenger Focus if the TOCs refuse or fail to resolve the situation.
First things first. The arrangements for through ticketing and rail retailing are specified in the Ticketing and Settlement Agreement (TSA). Each train operator’s passenger licence requires them to be a party to, and comply with the TSA. Franchise agreements also require operators to comply with the TSA. The TSA does not vary by train operator.
If there was to be a breach of the rules regarding impartial retailing, the location of the rule being breached, would be be within the TSA. After a thorough search, there is one clause which FCC may possibly be breaching but unfortunately it is not clear cut:
TSA said:
(3) Requirement to offer a full range of Fares
(a) If an Operator offers a Fare for Sale at a Station at which it is the Lead Retailer or on a train or at an Internet Site or at a Telephone Sales Office or at a Site that is an Impartial Point of Sale, it must also offer for Sale all Fares relating to that Flow which have similar Rights and Restrictions, including those which entitle the Purchaser to use other Operators' trains.
(b) Paragraph (a) above applies to Self-service TIMs as well as staffed points of sale, but not to any Self-service TIMs to which the Authority agrees it should not apply. It also applies to a Self-service TIM located at a Station which is operated by an Operator that is not the Lead Retailer at that Station unless it is made clear to members of the public that such a range of Fares is not available from the Self- service TIM.
(c) Any written information that is displayed on a Self-service TIM to which paragraph (a) above applies must be impartial between the Fares of different Operators.
So what the above basically states is that FCC is only obliged to offer the full range of tickets at stations where they are the lead retailer (being the lead retailer of a particular station should not be confused with being the lead operator of a particular flow). So basically what we now need to ascertain is whether FCC is the lead retailer of Stevenage and to do this we must reference Schedule 17 of the TSA, where this information is located. When we do this we find that FCC is the lead retailer of Stevenage.
Now I realise this situation has arisen before with Southern specifically at Three Bridges. At Three Bridges again Southern are the lead retailer Southern and therefore they have and obligation to offer the full range of fares for all journeys originated from Three Bridges.
I realise the above states the authority (i.e. the DfT) can exempt specific self-service TIMs from offering the full range of fares but as far as I can ascertain this is not the situation in either case. Schedule 17 lists, in addition to its information regarding lead retailers, derogations to provisions of the TSA wtih regard to retailing and neither FCC or Southern has derogation listed at either Three Bridges or Stevenage.