The law changed a few years ago on this 2011 I think.
It’s called continuous insurance where the car needs to be insured or if no insurance held has to be declared sorn.
As the op swapped insurance to another car and didn’t declare sorn on the rover this is why the letters been received. If you declare sorn that should be the end of it.
https://www.theaa.com/driving-advice...ce-enforcement