The only way we will know is if he tells the full truth, which is unlikely. If SE sat in the back of the hire car for the entire journey to Kent, she will likely have spat, kicked cried, etc which would likely leave a scuffs and dna on the seating, at least the trace of a struggle. Although this could depend on if she was tied up or incapacitated. Traces of what is left behind could maybe be an indication of how long she spent in the car.
I think it’s likely he didn’t stop anywhere near London anyway. I have a feeling it all went down at that site because he knew it would be empty. Although I find this an absolutely disgusting and terrifying thought, I think he will be well aware stopping in multiple locations would increase the risk of getting caught.