For "illogical" routes like LHR-ATH-DUB, ITA Matrix is unreliable with availability.
and entering random dates for FCO to LHR route A3+ gives various fares from ROM to LON which have rules such as NO DIRECT FLIGHTS ALLOWED and STOPOVER IN ATHENS PERMITTED AT EUR 75.00, indicating that this is one of the fares the OP is looking for, not just a combination of two fares.
Here is a random example for such a through IATA oneway fare not combining two fares but a proper LHR-ATH-DUB ticket.
