A bit of a crash...

That would be insurance fraud. Your problem now is that you have posted your intentions on a public forum so you now don't have plausible deniability for any missing parts. I have genuinely had a car interior stripped whilst at an allegedly secure compound waiting for an assessment from a loss adjuster, the fight I had to get it back to how it was was not even close to funny. So going the other way and saying stuff was there when it wasn't will be impossible especially as you have expressed an intention to defraud. Take many photos with date, time and location of how the car is when it leaves your possession, you will need them later.

Ok thanks. I'll put everything back then!

If it was written of as a cat C or more likely cat D the insurance company will be looking to sell it on to recoup some of their pay out.
I don't think the repairs were that bad it was more a case of you getting a decent value so I expect the car will be sold on and you might even see it on ebay in the future??
As such as near standard will be required and they may be looking for a standard car.
Removable aftermarket accessories would probably be ok and if you have fitted a better radio refit the genuine one if you have it.
Personally I think you have had a fair offer??
What are you going to buy with it??

Complete double standards, haggling over the cars value then removing the things that were considered in the valuation.... You can't have your cake and eat it!

True, true. I'll refit everything.