I'm the furthest from an expert but looking at the NA/LHR routes over the years for us I can say from what I've seen
1) there are no real rules/patterns and its a big secret.
2)355 days out seems to be when an initial set get released
3) about 310 days out a whole new set tend to get releases , we've just seen this as June 2014 have just had a flood opened
4) quite often a week or so before gets more
Keep looking (daily even) if you see something book it as you can always change.
And finally over peak revenue times there maybe very little
And not on all flights