That would have been awesome.
Mr. I wish I had an M5 wafting about the country lane "ahhhhh, an arse!"
Perhaps his previous car was an E39 M5 and like most people the time came where he had to be a bit more realistic with his next car, and he then ended up with a newer 5 Series? There are loads of M3 and M5 plates on diesel Range Rover Sports, 130i's, etc etc after this sort of thing has happened.
I doubt he honestly gave a stuff as he watched your shed fly past him. He was comfortably cruising along listening to the radio.
Oh and by the way, M5 BEX is fitted to a V8 545i with well over 300bhp.