My position has not changed.
http://www.nationalrail.co.uk/times_fares/offpeak.html
"
Buy any time, travel off-peak"
You can buy an off peak ticket at any time, some station staff refuse to issue them (which is incorrect) but if they are concerned you may use it early they can stamp on it "Restrictions advised" or write something like "Valid after 09:30" etc.
I'd be asking them who has told them not to sell the tickets, I'd be asking for names and I'd be asking if they are aware of ATOC policy and if I didn't get a satisfactory response I'd be writing to ATOC/Passenger Focus.
Nothing surprises me with SET. They seem to think they can treat customers like crap, especially where ticketing is concerned (we seem to have more SET related issues of mis-selling tickets or guards refusing to accept valid tickets than any other operator at the moment, many of these problems involve HS1). In another thread someone was asking if they should lose their franchise. I'm inclined to say yes, they should.
A good one to test them on would be try to buy an Off Peak/Super Off Peak ticket to somewhere like Peterborough/Rugby (or beyond) - these are valid "by any train" to London, to connect with onward trains from London to which restrictions apply (although with a railcard there is no restriction on Virgin). If they fail that test, that's a serious breach of the ticketing & settlement agreement I think!