I know the guys at Bramely quite well too, and regularly go and visit and indeed they do always have an amazing selection of cars.
A friend of mine actually bought his DB9 Volante there 18 months ago, and like the OP, this one only had 300 miles on the clock. The original owner bought it with his pension fund, but quickly realised his stupidity and sold it!
A couple of recent ones stories from Bramley stand out:
A wealthy banker who wanted to stand out from his mates at their shooting trips decided to buy their DB5 Shooting Break (similar to
this one), at about £300k, that's quite some toy.
Again, similar to the OP, they recently had a 300mile BMW 735, 3 years old with all the toys. He told me that it was bought to drive the owner around, but was simply never used.
There are loads of people like this.