The best I can do is PHL-FCO on AA, sold as BA1575, followed by LHR-IAD on BA217 or BA293 (both on the 747), which comes out at $1,193 per person. From Washington airports it's about $400 per person higher, and a lot more hassle for connecting through London. Depending on where you're based, I'd either drive to IAD and get a one-way rental car to PHL, or take the Amtrak from Union Station to PHL.

On the other hand, Aer Lingus (with one connection for PHL-DUB-FCO) is $885, returning LHR-DUB-WAS. Up to you whether it's worth connecting to save $300 per person.

VCE-LGW is currently €80 one way including a checked bag on BA.