The response to that level of BS is MCOL. Sue them every bloody time. £35 court costs payable by them. Eventually the message will sink in.....No doubt when you eventually get an email back from TfW. The customer services will say the staff member was correct with the first response (after-all, railway staff never make mistakes), and as a "goodwill" gesture will refund you the £4.25 ticket just this one time.
No mention of refunding the extra ticket you were forced to purchase due to the staff being poorly trained (I've had experience with the staff at Shrewsbury when they tried to claim a "Explore Wales" rover wasn't valid there as "Shrewsbury is in England"). If the railway industry was focused on providing a good customer service for when things go wrong they would offer you decent compensation to say sorry, i.e. £50 cash.
Either that or the response will completely ignore any points mentioned, and go on about something about Covid. Or will blame whatever website you used to purchase the ticket as being wrong (assuming it wasn't purchased directly from the TfW website)
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AbsolutelyIt was definitely breach of contract. In addition to the minimum fare not applying to advance tickets, the advance ticket was sold in conjunction with an itinerary so should have been accepted on that basis too.