A3: LHR-ATH-WAW-ATH-LHR, $167, 23Hrs total, 4,990 miles true distance, 4.4cpm for A3.

(I haven't checked booking classes, but I assume U,P,T, earning 75% on A3 or 3,742 miles).

Available on most weekends/Saturdays in March. Available also WAW-ATH-LHR-ATH-WAW, 23hrs total, same price.

If prices appear higher on google flights link below, click through to an OTA it will likely reprice down to $167.

This combo gives you all 4 of the required A3 segments for renewing *A Gold in one day.

https://www.google.com/flights/#sear...LHR0A3608;a=A3

https://www.google.com/flights/#sear...LHR0A3608;a=A3

https://www.priceline.com/fly/#/chec...60:A3861-A3608

https://www.priceline.com/fly/#/chec...08:A3609-A3860

You do three of these and you have 11,227 miles, only 800 miles shy from renewing *A G for the year with A3, for $500 total.