Hey dude,
When I had a 309 which I used to rally the hell out of on the off road tracks near mine (as much as I love Golfs, the 309 is a pretty addictive drive, gets better the harder you push them lol), I used it as a commuting car aswell, and was unfortunate enough to have a Vectra plough into the back at me at full speed. Ironic thing was, I was stationary at the time, waiting to pull out at a junction, and so he should have seen me, but no, 40MPH full force into my boot, straight in the crumple zone, car was written off and towed away same day.
As the car was one I used to mess around with under the hood (or at least try to lol), I told my insurer it was worth £500 (I picked it up for £200, so I was happy enough with that), and I had a zero excess.
The car, as I said, was towed away, and after they took the hire car from me a week later when I'd bought a replacement, they told me that it was up to me what to do with it, do I want to keep it, or can they keep it, even though it was a full write off.
So I think it must vary from insurer to insurer, or maybe it just depends on which repairer the insurer use, at the time I was with IBuyEco, and the repairer they used was Karl Vella's Preston branch (the one local to me for some reason got ignored and they chose one miles away, no logic with insurers I guess lol).