As well as normal off-peak fares, there are often off-peak day return fares and super-off-peak day return fares available. Further, the number of passengers can be relevant for off-peak travel (as groups of 3 or more get a third discount on many routes). Further, using two 'Advance' single tickets can be cheaper on certain services. So the person taking your call needs plenty of information if he/she is to be able to provide an accurate answer to your query.
It's far simpler to look for yourself, here:
http://www.nationalrail.co.uk/
(Remember to use the 'More options' button to add your railcard).
I get frustrated though in fast food restaurants, where the staff often seem to be unable to work beyond the script. e.g. (in KFC):
Me: "A three strip meal please, to eat in, with a cappuccino and sweet chilli sauce"
Them: "Is that to eat in or take out?"
Me "To eat in, please"
Them: "And what drink would you like with that"
Me: "A cappuccino, please"
Them: "Would you like any sauces with that"
Me: "Yes, sweet chilli, please"
GRRR!!!