Generally insurance is not an issue for a Cat C or D write off.
You do have to declare it to insurance companies but most take the view that as long as it has a valid MOT since being a write then this is ok.
For example, with Greenlight, you will declare it is a previous Cat C (if you did not know that it had previously been a write off, our system will let us know when we enter the reg) then we will give you a quote, which will be the same as it would be if it was not a write off. Sorted.
In the event of an accident, you would get a partial value back for your vehicle so basically you would get a percentage deducted as it has previously been a write off.
Hope this helps and good luck with the quotes, you are welcome to pm me your details to give you a call?
Dan @ Greenlight