Silver you say, probably a police advanced driver :) Before anyone takes offense I know two who drive like that, one retired and one still active, wouldn't get in a car with either of them if they were driving!
This wasn't advanced! Deffo not an undercover police car. funiest thing was about 20 mins later I passed him - just shows how far pushing to the front actually gets you. honestly though - this chap was a total pillock! We all do stupid things on the roads now and again but this was the absolute pits, never seen anything like it!
I see it every morning on the M6 (around J3 to J6) there are some idiots out there. I've been know to stay in the middle lane too long and push in nearer to the junction when late for work, but undertaking like that is just stupid, and like you say it doesn't get you there any quicker.