I would suggest that twitter responder is wrong. The industry's own code of practice (CoP), and Consumer Protection Regulations (CPRs) consider price to be "material information". Omitting important information, in this case the fact that there is a cheaper product available that allows the same journey, could be seen as both a breach of the CoP and CPRs.
http://www.nationalrail.co.uk/static/documents/content/RetailInformationForRailTicketsCoP.PDFAnd then there's
The Ticketing and Settlement Agreement, adherence to which is a franchise requirement, which says:
6-30 THE IMPARTIALITY OBLIGATION
(2) Specific requirements
(a) If more than one Fare is available that meets the requirements specified by a potential Purchaser and he does not specify which of the Fares he requires, the Operator must seek any additional information from him that is necessary to enable it to recommend (in an impartial manner) which of them is suitable for him.
(b) Where an Operator is asked to recommend a suitable Fare or paragraph (a) above applies, it must request sufficient additional information to enable it to make the recommendation. This may (for example) include any of the following:-
(i) the departure and/or arrival time required;
(ii) how important it is to the person requesting the Fare to minimise the journey time involved;
(iii) the importance to him of the price of the Fare;
(iv) whether he minds changing trains;
(v) (if a return journey is to be made) the extent to which he needs flexibility in his choice of trains for that journey;
(vi) whether he wants the flexibility of an Inter-available Fare; and
(vii) any special requirements that he has.
(c) If more than one Fare is suitable, the Operator must explain the main features of the alternatives in an impartial manner.
So if, for example, you are at Redruth ticket office after 0900 and say, "I want to go to Plymouth, coming back later today", you
must, at the very least, be offered the Ride Cornwall Ranger.
I'd go further and suggest that if you ask for a day return you should also be offered the Ranger. If a specific product is asked for then that can be sold without further interaction. However, "day return" is not a specific product.