Same here, my house insurance became due in February. They more than doubled the premium and put the rebuilding costs as £600,000, on a house that had been only recently valued at £300,000! It seems that they took one look at the postcode* to calculate their figures. I did no more than shop around and got a deal for less than the previous years premium for exactly the same insurance.
*Some houses in this area are worth a million apiece or more.