The National Rail Conditions of Carriage have this to say on the matter:
"Before you travel you must have a ticket or other authority to travel which is valid for the
train(s) you intend to use and for the journey you intend to make.
If you travel in a train:
(a) without a ticket; or
(b) the circumstances described in any of Conditions 10, 11, 12, 18, 19, 22,
30, 35 and 39 apply;
you will be liable to pay the full single fare or full return fare or, if appropriate, a Penalty
Fare (see Condition 4) for your journey. You will not be entitled to any discounts or special
terms unless either:
(i) at the station where you started your journey:
there was no ticket
office or no ticket
office was open
and
there were no
self- service ticket
machines or no
self-service ticket
machines were in
full working order
and
in Penalty Fares areas
you bought a Permit
to Travel unless no
Permit to Travel issuing
machine was in full
working order
or
(ii) the notices and other publications issued by the Train Company in whose
train you are travelling indicate that you can buy tickets in that train.
In circumstances where (i) or (ii) apply, you only need to pay the fare that you would have
paid if you had bought a ticket immediately before your journey.
Special arrangements may apply if you are disabled. You will find details of these
arrangements in each Train Company’s ‘Disabled People’s Protection Policy’.
For the purposes of this Condition, and Conditions 4, 39 and 41, “full single fare or full
return fare” means the highest priced single or (if requested by the passenger) return fare
appropriate to the class of travel for the journey you are making."
http://www.nationalrail.co.uk/system/galleries/download/misc/NRCOC.pdf p4-5
During booking office opening hours at Wigan NW, part i does not apply, and judging from the discussion, I think it safe to assume that part ii also does not apply. Therefore, passengers boarding without a ticket would "be liable to pay the full single fare or full return fare". This is even clarified, not that I consider it necessary, as "the highest priced single or (if requested by the passenger) return fare
appropriate to the class of travel for the journey you are making". Therefore, I think it is quite clear that the NRCoC compels Virging to sell Wigan-Preston day returns on the train as they are the only standard class ticket type available for that flow. In addition to the Virgin passenger charter stating that Virging comply with the NRCoC, the following paragraph is a part of the introduction of the NRCoC:
"It is a condition of the Passenger Licence granted to each Train Company by the Office
of Rail Regulation that these Conditions apply to tickets sold for journeys involving its
services and those of other Train Companies. There is a list of the Train Companies in
Appendix C."
The list in appendix C contains: "West Coast Trains Limited (trading as Virgin Trains)"
I think that is fairly conclusive.