On SkySkanner, I see:
- Virgin: LHR-SAN (via DTW) and return (via SLC), "operated by Delta": £580
- Air France: exactly the same flights, "operated by Delta": £956
...and there are other combinations too at different prices.
Even more oddly, if I restrict the search to Delta only, I don't even see that particular flight!
You can see it for yourself, using the same Skyscanner search.
The actual flight I identified is:
- LHR 09:25 to SAN 17:35
- SAN 13:16 to LHR 10:50
I don't understand how this particular case of operated by/marketed by works, or if it is supposed to work like this. I am also puzzled about the implications of these arrangements for the various loyalty programmes: On what tickets or flights are Flying Blue XP earned?