Aegean does not file any fares for FRA-LHR or v.v. - which is unsurprising, as it is not a routing that would make much sense for the overwhelming majority of fliers.

As such, you would end up having to piece an LHR-ATH fare together with an ATH-FRA fare.

Somehow, I don't think you are going to find any really competitively priced tickets for such an itinerary.