OP, what colour is the "unmarked police BM estate sat at 70mph in L1".
If the Golf driver knew it was a private car/family then he was a moosive "insert appropriate expletive" - as could only have been goading you.
I drive 30K a year, mostly on a mixture of motorways in my car(s) and despair at the fall in standards/roadcraft. From yo-yo speeders through to lane hogs to indicate-and-move out plus sit in blindspot merchants, no idea what goes through these drivers minds.
Hindsight is great, perhaps next time either drop and hold 60 or move up a few mph. As someone who drives 60K/PA, sure that you more than experienced to chalk this up.