Petition Force insurers to pay insured value if insured item is written off / lost / stolen

Almost all policies are 'up to a value' for example
insurance for postal services, car insurance, etc.
BUT
premium is charged from INSURED VALUE and insurers pay what THEY estimate (AS LOW AS POSSIBLE) and in many cases they refuse to pay because one can not provide oryginal sales receipt (sometimes not available when for example part of bigger thing was shipped and receipt is for whole item).
this should be BANNED because it is not just that we pay full value and in case of loss receive PEANUTS or NOTHING. for example one forgot to take his laptops' power supply when visited family. family sent it to him via Royal Mail. they LOST item and because there is no receipt they refused compensation (receipt never existed because power supply in question was sold together with laptop).
Another Example: car insured for 5000GBP. Written off and insurer pays 3k because this is what THEY estimated BUT THEY charged premium for 5000 knowing what the car is.