I have absolutely no idea, but I would presume that since the units attacking and offering support are French, England's army in London would indeed be dislodged. however, I can't see why England would agree to that maneuver.

To my understanding it doesn't matter the nationality of the convoying unit. As long as the convoy isn't dislodged it can convoy any attack.

That would be an interesting one to plug into one of the order resolution packages...

Yes, you can convoy an enemy unit into dislodging your own unit.

The specific examle you described is actually the exact one used in the test case
http://webdiplomacy.net/datc.php
6.D.16

But can anyone explain to me why anyone would agree to do this?

Easy, a trade of SCs... The retreating unit (say it was Edi not Lon) could retreat to Lvp. Maybe Lvp is owned by France. Or maybe it's France and Germany on England's SCs and it's spring and they want to get two of England's SCs but England could try to move in if Edi moved out of it's own accord and cause a bounce. By dislodging Edi if they are successful, they guarantee the loss of both Edi and Lvp, but if England manages to bounce it, they still keep Edi which at that juncture is more strategically important to them.

Ah. That makes a bit of sense.

Also to destroy a fleet so the unit can be rebuilt as an army or vice versa. In that case, do it in the fall but not in a supply center.