Marked plate of doom - potentially over

I’ve had this car for a while now and phoned up multiple times and was told the marker would be removed. I’ve only used it a few times since the last time I was pulled which was a couple weeks ago, and I’ve been intentionally travelling mainly through backroads/residentials in order to avoid major junctions and potentially anpr’s. Went out today to drop it off to my mechanic and get a few bits done just before I sell it, mechanic was done by 10am and I started driving home.

I assume the marker has been removed as I’ve been unbothered for a couple weeks now even though I’ve driven past police cars. This is until I realise when moving off at a traffic light the car behind me lets a silver fiesta overtake it, I noticed the fiesta is an unmarked and after turning a bend I got a good look behind me and realised the one that let him overtake and the 2 behind it looked like unmarked cars aswell. I turned into a quiet side road and pulled over to minimise disruption to main roads and they all sort of surrounded my car. I got out and spoke to them and as they’re doing their checks a police van arrives. All much more nicer, credit to them than the tossers who chucked me in cuffs even though I was fully compliant and pulled me about last time. The driver of the van introduced himself as a sergeant and I told him about this marker etc. he apologised and said he’d get it removed for me. Hopefully he follows through but I decided I’m going to sell it soon, let customer know about this marker and show them proof I’ve requested to have it removed.