Basically LHR to any Greek airport via ATH will be less than €650 except when booking last minute, or some exceptional circumstance where all flights are filled. Just pick one. It can be done in less than 24 hours especially if you take the overnight LHR-ATH, though I would always prefer to spend a bit more time in the Greek city.
But you have 11 months. If these are your only options to renew, this means you believe that you won't fly Star Alliance at all besides the trip you are booking.
If you do intend to fly with *A more this year, then why is paying €650 extra for exactly the same product and service even a consideration? Is Gold that you wouldn't even use worth €650? Obviously it's a lot harder to get back to Gold if you lose it now, but you can buy lots of food and drink, pay for fast track many times and buy lots of extra baggage allowance with €650.
In past years I have renewed my Gold for not much more than €650 and most of my flights I would have taken anyway even if they earned 0 miles. Though with SAS earning gone this won't happen any more.