The other thing to read with GFV's is that they should be a fairly indicative price of what you could sell your car for at that age/mileage.
I was also told btw that every thousand miles extra on the deal would be a fiver a month.... I didn't see what the over mileage charge was but thats £235 for a thousand miles... probably be cheaper to pay it (my VW one is 10p a mile).
Ive given up telling them what actual mileage I do and now just take the car as 8k miles per year to keep the price down.
I went under with one car at 12k miles per year and they don't give you the difference back.
So the next car I put 10k and went way over it. Come trade in time the extra mileage didn't matter a jot. The difference on WBAC for quotes with different mileage was negligible.
For me it will only really come in to the equation if you hand them back the keys and walk away, which I doubt I'd ever do.