Seat Blocking...
Hello Bill!
I operate ATH quite often, and just try and get a seat as far to the front as possible to disembark as quickly as possible.
As of late, the early ATH-LHR has not been the busiest of flights, and due to the weight and balance restrictions, the Euro Traveller cabin has the majority of the Mid Cabin blocked off. So, if you can, make sure you try and get a seat as far forward as possible.
If you are indeed sat in the Aft cabin, you will not be permitted to move forward before take off, but will after take off.
I am presuming you are traveling in Euro Traveller, but could be totally incorrect...!
Have a good flight!