As Fivejuliet has pointed out the a/c swaps are done in AMS (LHR not so regular although it does happen on rare occasions).
Not to be prudent but referring to individual a/c in the fleet as oldest? The oldest Airbus A320 is from 2000, 14 years? Nothing and how you're judging the technical status of each a/c by its age is well, nonsensical. By the way VA and
VB are great birds as is the whole fleet, both just had heavy maintenance done during the winter (D-Checks).
Regards you wanting to see VM, I'll post when its in the system for a swap (be advised it might not be down for a few months).