If they offered you a write off amount, then the value of the car according to the insurer is that amount plus your excess, if you are liable to pay the excess on the claim.
The cost of the hire car is a factor in the write off decision. If the cost of repairs is £3500, the cost of the hire car is £250 a week, and the estimated time for repair is 8 weeks, and the car is worth £6000, they'll write it off because wiuth the cost of the hire car considered, its not worth them gambling on the repair taking 8 weeks or less, especially with supply chains still being a bit ****.
Insurance doesn't provide a bottomless pit of money to cover claim related costs.