I'm confident you would have access to the Aegean Lounge. I've flown through Athens several times and always accessed that lounge - but to be fair I can't recall which carrier I was on each time.

The Star Alliance Lounge Finder says "As an International Business Class Customer you have access to any Star Alliance member carrier’s owned Business Class lounge at the airport where your flight departs". I do notice that the BA Terraces Lounge is a TK contracted lounge and I'm sure I've never been there.

I really enjoy the Aegean Lounge so grab this one if you can. It's one of my favourite lounges despite it's small size. Very good food and a decent selection of interesting things to drink.