got flashed - not sure what it meant

The other day I was driving down a dual carriageway and had a flash of blue lights however have no idea why.

It was about a 5 mile long dual carriageway and I was in the left lane doing about 80mph - it was quite late in the night and the road was empty. I was about 1 mile into the dual carriageway and had some cars overtake on my right as normal.

Suddenly another car came behind me and I don't go over 80mph so I thought if the car got fed up - it would over take me in the right lane like all normal cars do.

The car continued following me and then suddenly blue lights flashed on the car so it seems like it was an un-marked police car (I don't know - couldn't see car properly or passengers.)

I then panicked and dropped my speed to 70 then 60 and just continued panicking and thought about where to pull over. As it was a dual carriageway - there is no hard shoulder so I didn't want to get in further trouble for pulling over in a dangerous spot.

So I thought lets continue until the roundabout at the end and then stop where it is safe and legal. I was under the impression that I was getting done for speeding.

During the middle part of this dual carriageway - there is a slip road to the M5. When we passed this junction, the "police car" behind me took the exit and continued at their normal speed.

I was left soooo confused by this encounter and just continued with my journey afterwards but what the hell was that all about?