Do you have to fly from LHR?? Aegean are starting STN-ATH on 15/05, are a lot more pleasant and reliable, and you can get M&M miles for their flights.
I have never flown OA on the LHR route, but my understanding is that they usually fly widebodies on the route, so the seating in business is good. Food is the only strong point of OA, so you should also be fine in that department.
On the other hand, you can forget any pleasantries in terms of service- don't be surprised if the FAs don't use unnecessary expressions such as "thank you", "please" and the like...Their planes are not famous for their cleanliness, and punctuality is also very relative...And that's assuming your plane will turn up (though I think the LON route is a bit better than other ones e.g. to JFK).
I have heard good things about the ATH lounge, but I've not been there.
From LHR, they fly out of T2 and use AZ employees for their check-in, so I expect that they will use the AZ lounge which is basic but does have nice coffee and outside views. The above assumes that AZ will still be in business by the time you fly! OA should have shut down decades ago, but that's a story for a different day.
Personally, I can't see why anyone would fly with what arguably is the crappiest "full service" (Jesus wept!) airline in Europe if they have an alternative choice. On the LON-HER route, you have two excellent alternatives; A3 from STN or OK via PRG, use them for your peace of mind (and miles).