If you are lucky enough to prove you did inform them of points, you will only have to pay the £250 +£200 excess and pay off the finance, they will offer you market value.
If you can't prove it, they may not pay your claim, you will still have to pay the finance. They may also chase you for extra premium in respect of the increase they would have put on for the points.
In respect of hitting a car in the rear, 99.9% it's your fault.
Circumstances alters cases!!