What do solid blue lights on a police car mean?

Posted by LawabidingKhajiit@reddit | AskUK | View on Reddit | 50 comments

Currently a passenger and on a dual carriageway, doing about 68 and overtaking a truck a police car sped up behind us with just the very tips of their emergency lights on, a solid blue. Once we were past the truck and moved left, it passed us and turned them off as it sped away. No flashing or sirens at any point.

Is that just a 'making sure you can see me hint hint' signal? Don't remember anything from the highway code about it.