Some of them (for example British Airways) seem to release dates 355 days before flight date, but then if you want to book a return flight you would need to take that into account, as you could get an ourbound date but not a return.
With the LOCO's it can be anybodys guess, easyjet in particular are a bit erratic, and they may list one airports departures to a given destination, prior to anothers. ie Luton before Gatwick. Although its tedious it can be best to check every day if you want a good price.
You might be better off using a screen scraper like moneysupermarket.com, just put your dates in and see what it brings up. They dont pick up all airlines but they list quite a few.