The problem with trying to get it done yourself is that it is going to be expensive. The insurance company should have repair contacts outside of BMW dealers. The car is your property and you should insist they repair it or try to negotiate a settlement in lieu of you getting it repaired.. There are independent BMW specialists and also BMW specialist breakers who hold large stocks of second hand parts. Like someone else mentioned here it sounds to me that the quote of £17k was either the whole wiring loom or they did not want the job. You need to go up the chain in your insurance company to their assessor who inspected or will inspect your car. Keep on to them as most have no sense or urgency. Also try ringing a few BMW dealers explain what as happened and try to get an estimate. Also try Auto Electricians. It is repairable. If insurance do pay out they will auction it and someone will buy it to repair.

If the OP chooses his own repairer, where does that leave him if they can't fix it.

IE there may be damaged electronic modules (which were shorted out) when the wiring was cut. No doubt a new instrument cluster would need to be coded to the car (on a BMW even the battery needs coded!).

If you don't go with the insurers repairer and your own repairer can't fix it, I think you'll find your insurers will wash their hands and leave you on your own. I would stick with your insurers repairers.

Where did this happen? If it was outside your house. I'd be changing the car for something less desirable. If you get the BMW repaired, who's to say that the same thing won't happen again!

The concern for me here would be that the OP would have to do the deal with the insurer and buy the salvage before he could get the repair done. It's not like a few bashed panels, who knows what the damage is and whilst he could get an estimate a firm quote might be more difficult to obtain. And it's the sort of thing that might prove more difficult than it first seems. If the repairer can't get it it working, how much further do you go with parts bingo?

If the OP can find a cheaper repairer acceptable to the insurer (in place of a write off) and leave the risk with the insurer, that would be preferable.

I think I'd just leave the insurer to do their thing, and if they write it off be prepared to haggle. This car could be troublesome for the rest of its life if there are damaged components.