We’ve rented through Costco a few times. Each overseas time the rental company was Enterprise. Fuel for a week will be nominal and will depend on how far you drive. When renting through Costco you need to get insurance at the rental counter if not using credit card coverage.
There are basic coverages up to a zero deductible coverage. There are also optional coverages to cover windshield or tire damage as well as road assistance. If you use your credit card CDW basic coverage then other coverages offered by the rental car company, such as zero deductible, will not be available to you.
As you are going to use your CC coverage, your total price should be £463.16. You will be billed in GBP so the total in USD will depend on the exchange rate at the time you pick up the vehicle.
Not sure when your trip is, but keep checking the rates. Since there is no cancellation fee, if the rates go down, make a new reservation, then cancel the original. I’ve done that on a few trips and each time the final rental price was about 1/2 of the original price.