I'm guessing a lot as i don't have direct experience... I do have a couple of clients that occasionally do insurance repairs and from the discussions i've overheard i suspect it's as likely about them expecting the insurance company to try and bully them/twist their arm over price (putting the price up gives them room to manouver if/when the assessor started questioning things) as it is about them looking for a bit of extra profit. Similar deal to insurance companies trying to bully/imply you have to use their 'approved repairer' (read, they've negotiated good rates) when you don'tAnytime I ask for a repair quote at a garage the first question is "Are the insurance paying" - if I say yes the price immediately goes up.
My daily driver kinda follows the bangernomics principle, certainly cheap enough that most knocks would make it a class D write off to an insurer. For this reason i typically opt for the largest excess i can (telesales muppet got very confused with this as the excess wasn't far short of the cars value) because excess is meaningless in a non-fault claim and a claim on my policy is going to cost me regardless through increased premiums so i just won't.
If a claim on your policy will result in an economic write off you can usually buy the car back to fix but as this is a fault claim you're going to pay regardless so might as well just fix it
It appears motor insurance is currently being looked at...
http://www.lovemoney.com/news/cars-computers-and-sport/car-insurance/12804/government-crackdown-on-high-car-insurance-premiums
http://www.bbc.co.uk/news/business-16175022
I don't get the £20 thing unless it's a case of 6% or £20 (obviously whichever is the bigger number)? My car insurance was renewed a couple of months ago, policy, legal cover and 6% tax itemised seperatley but that was it. I can't see the insurance companies voluntarily hiding it