Pre-Schengen airport design was based on entirely segregating arriving and departing passengers. Terminals within the Schengen area built or heavily rebuilt after Schengen started do indeed feature more free movement. However I've yet to find one that allows someone who has arrived from outside the Schengen area (remember the OP is coming from LHR) to get into the airside Schengen departures area without a boarding pass. There are certainly airports, perhaps AMS is the best known, where many of the facilities are landside and can be used without being an air traveller at all.
Athens airport was designed and partly constructed before Greece joined the Schengen area, though it didn't open until two years later.
Ah, I didn't realise that you were travelling as late in the year as November. Sorry! This is indeed in the winter timetable period.
By November the weather is starting to turn to winter too. Compared to high summer, when it hardly rains at all, November in Athens averages 11 wet days, so it might be a good idea to have alternative plans for wet or dry first days so you can make the most of a good day or avoid getting soaked on a bad one..