I'm not a massive fan of the fact that this thread has turned into a place to seemingly escalate customer support issues, but:
The website must have offered me London Euston as my destination and when a ticket came up with what I thought was a good price (£9.40) I clicked on it.
I've checked the search logs; "London Euston" was specifically searched for, and picked as the destination:

(NLC 1444 is London Euston)
The journey planner then planned you a journey from Solihull to London Euston and sold you tickets valid through to London Euston. You then wrote to customer support and complained that the site had sold you tickets valid for travel to the destination you entered into the search form.
it would appear that an opportunity to check the suggested itinerary and ticket breakdown was not taken during the search process, on the ticket information page, or on the checkout page where the tickets are summarised before payment. Similarly, an opportunity was not taken to request a refund during the fee-free same-day void period.
And I am now left wonder why a website, which makes great claims about saving the customer money (and usually does), charged me £3.00 for a single Zone 1 underground trip, which costs £2.80 (peak) or £2.70 (off peak) if I bought it separately using contactless. I suppose they would claim that it's cheaper than the £6.70 paper ticket between two Zone 1 mainline stations.
Third-party retailers cannot sell TfL's fares. They must either apply the add-on fares documented in the internal knowledgebase (table below) - rewriting the London Terminals fare to include travel validity within the required zones - or else sell a paper travelcard. Legs must not be left unticketed by default (i.e. retailers cannot just assume customers are happy to use an Oyster card by default). As you observe, a paper travelcard would have been more expensive than the final ticket you were sold.
These add-on fares are variations of a ticket through to a London Terminal. They cannot be presented as a separate ticket, because they are not - the TIS is selling one ticket from Solihull to London
including Zone 1.
I felt it was RailUK's website that was at least partially at fault
I had worked out by then that it would be a Chiltern train they would be travelling on, but just thought that was how they described their ticket to London (after all, Marylebone is in Zone 1).
I appreciate you feel that the website was unclear. The ticket destination was shown as London Underground Zone 1, which means the ticket includes Underground Zone 1 validity. It would be entirely misleading to use the terminology "London Underground Zone 1" to describe a terminus station (London Marylebone) where the ticket doesn't have validity in London Underground Zone 1. In fact, I'm quite confident a ticket issuing system doing that would fail its accreditation.
No customers should be being made to "feel stupid" after purchasing from the RailUK Tickets retailer (or indeed, any Raileasy site), and it's acknowledged and well-understood that customers make mistakes every day. Customers receive goodwill gestures on a regular basis, even where they have made mistakes themselves, but sometimes it's the case in life that we screw something up and have to take repsonsibility, figure out what it was that we got wrong and sometimes swallow the financial loss for the mistake.
I don't think the customer service rep's response to you was disparaging or derogatory; although it could've included an apology for the inconvenience of no refund being available under these circumstances.