I'm not sure if this is still correct but I think you can turn insurance offers down if they're lower than what it would cost to replace the car, and effectively demand that they replace the car like for like. If they struggle to do that, then you can ask for more. I had a write-off (not my fault) a few years back and rejected about 3 offers, as they were all ridiculous, eventually, I got what I would have if I had sold on the open market.
In some instances (especially now with the high demand), cars can be worth more than what you paid for them new, which should give you grounds to reject insurers' offers, as the offer does not replace the car in the same way.
Not sure how this works when you're claiming off someone else's insurance, but it shouldn't be a cop-out for them to offer you 10%-15% less than you paid, and maybe even a bigger gap to what you can replace it for immediately. I imagine if you threatened court action you would get a fair price, but I bet most don't do this.
I still took gap out on my car, as I was in a bit of a rush, and I suppose it gave me some peace of mind, but a bit of a fight would probably cover the same, if not more, I'd probably end up doing both.