Had an accident...What do I do now...?

[triumph2.5pi]

Hello all
I just wondered if anyone had any opinions or advice on something that happened to me today...
Someone sideswiped me whilst I was stationary in traffic at a very busy roundabout. Totally their fault. The front nearside bumper is damaged. I'm awaiting a written estimate, but a verbal one was "around £800". The guy at the bodyshop was of the opinion that that the car would "likely be written off" if it went through insurance.
It's a 2000 2.8 Sport, 106k miles, FSH, well looked after and in excellent pre accident condition. I don't know what an insurer would value it at? I suspect something very low indeed? The car is worth much more to me than to anyone else given the time and money I've spent on it and I'd like to keep it if possible, but obviously would also like it fixed!
My concern is that if I deal with this through the insurer, once that process has been started and if they did indeed decide to write it off and offer me a derisory sum for it, can I stop the process at that point? Or is that a genie that doesn't go back in the bottle?
I doubt the other driver is going to come up with £800 in cash to deal with it. Quite possible they won't even admit liability I imagine. Others must have seen the incident, but in the heat of the moment I didn't get any witnesses unfortunately, so it could turn potentially into a "He said, she said" scenario.
Any thoughts or advice on how to proceed would be gratefully received as I'm not sure what to do now! Thanks :-)