You need to wait until 3 months beforehand in my experience.
Using 23rd-26th May (random dates from F-M just as a survey) you're getting no Virgin AP fares at all, and it is cheaper to go NXEC. (£61 for a return, but you really have to work around NXEC to get these fares). The trainline don't seem to have Virgin AP fares which is odd...
My internet sucks (Virgin Media

) so i can't get either Virgin or NXEC to work.
EDIT: Working now
Virgin are marginally dearer with "Saver Half Returns", but NXEC have fares about £25 each way, give or take, through their own website. Some of the best fares are change at EDB, though.
Your best bet might be to go to GLC travel centre next time you're in, and query the presence of AP fares for the journeys you wish to make. Because you're going to London for a weekend, though, I'd imagine you'd have to be quick!