bought a car in 2018 without knowing it has an outstanding finance

So as the subject says, I've had a car since February 2018, actually bought it from my dad which I haven't talked with since April 2019, he had some drinking problems, we were fighting pretty often but I didn't expect him to do this to me, I've read some articles about innocent buyers and that I can keep the car if I bought it in good faith but I don't really know how to proceed and who to talk with, what solicitor to find etc. At the border the police or whoever they were (NaVCIS) told me that the car has an outstanding finance and that they have to keep the car as the finance is 3 months behind, they didn't give me too many answers, they treated me like I was the biggest criminal in the world, didn't let me get out of the car for 30 minutes. I haven't spoken with my father since April, I didn't even have his phone number, I managed to talk with my mother and I finally got my dads number, that police guy called him and my dad actually told him that he doesn't care, that yes I bought the car from him and that he didn't pay the finance since April just because he didn't want to, the police didn't care, they just took my keys out, told me to get my luggage and leave without the car, I tried explaining to them, talk to them, reason with them, they just didn't listen. I even got a full fuel tank, I had a ferry reservation and I lost that money too and I even had to get a cab back to London that was £180, even just made the MOT and had to change 2 tyres and rear brake pads, that was another £500 to get me going for the road trip across Europe. The guy from the frontier kept saying that the police was actually searching for my car, I told them that I have the V5 on my name, at my home address, why no one contacted me? He just said that they don't talk with DVLA, like what?!

I don't even know where to write or what exactly to say as I have too many things in my mind right now, hopefully I made myself clear with my situation and I'm hoping someone could help me with some guidance,advice, anything, please!