I think you answered your own question.
the cost of cars has gone up this year, with less being built and sold, having to reduce people on production line due to Covid distancing, meaning slower lines and less volume, but same fixed costs and redundancies to be paid for.
as you say the PIP has only gone up by the cost of inflation I think.
The AP is based upon the cost of the car, the sum of 3 years PIP, the residual value of the car.
The difference is the AP, so as the cost to buy goes up and residual values go down, the AP goes up abd/or better specked versions are dropped due to going over the price cap.
I will say this again, none of this years increases are because of Brexit! If any manufacturer says ut us, tgen that’s an excuse