yes, the cart.php and confirm.php files are in the root of the power store directory.
the code you posted uses the eCart WA_eCart_DisplayMoney function which cases the price to display using the currency symbol you specify in the eCart object.
I dont see anywhere in the cart of confirm page where the WA_eCart_DisplayMoney function is not used.
as long as the eCart object has been edited on line 366:
$WA_Store_Cart = new WA_Store_Cart_Definition("WA_Store_Cart", 0, "$", true, 2, 0, "oz", true, 2, 1, 0);
to use the currency symbol you wish, those pages should be fine:£
$WA_Store_Cart = new WA_Store_Cart_Definition("WA_Store_Cart", 0, "£", true, 2, 0, "oz", true, 2, 1, 0);