I have also been looking at how to get to ATH and my conclusion was to go to LHR. On ID travel there are lots of flights between the two. Alternatively, I think there might be a low cost carrier (Ryan air/ easy jet) that goes directly from London to Santorini. You might have to get a taxi to LGW, but cost wise it might be a wash versus getting to ATH and then ferrying to Santorini. Just an idea! ATH is always a tough one to nonrev- on AA you have to worry about weight restricting. Good luck!