The mileage limit is 20k a year.
I extended my last car after the 3 years with 60k on it.
Ended up I had the car 5 and half years and went back over 100k miles.
There is no hard and fast rule for extensions. People say you need low mileage but that isn’t my experience.
This was on a Kuga with an AP less than 2k. When I asked for the extension they mentioned the size , spec and price and as it was considered are higher value car they where fine.
If it’s a small base model hatch then it’s obviously harder for them to justify without low mileage as the residuals wouldn’t be good enough.