I used to get stopped a lot by the police when I was driving the mini. I think half the time they just wanted to take a look at the car as I was rarely asked to produce any documentation. The general question was have I got an MOT.
I was once asked by a copper where I was coming from. I told him work so he proceeded to ask me where I worked. I was standing there talking to him wearing a Wilkinsons polo shirt. I was half tempted to say Tesco but thought better of it.
I used to always drive past the bikers night on a Thursday at the Stormy Petrel on the A41. The police were always there. I think they used to get a bit bored so always went for any easy target just so they had something to do. Its probably a lot easier to get a 998 cc mini driven by a 17 year old to stop than a Yamaha R1 driven by some nutter who may decide to take his chances.
My advice is to co-operate and stop. If you keep getting stopped in the same area by the same copper then I would register a complaint to his station about harassment. If they have no reason to stop you they shouldn't keep doing it. As you are in a mini it should be a fairly distinctive car so when they see it again they should know not to stop you again for no reason.
As soon as I bought my escort the police left me alone (apart from the time I slid across double white lines due to oil on the road).
40 years ago when my dad used to drive his mini van the same thing happened to him. The problem isn't new. He used to get stopped all the time and be made to explain why he was carrying tools in the back of it. He's a building engineer but the police never seamed to believe him.