Your shipping rule looks like it is setup correctly to charge 75 if the total is greater than 500. When I try to test the rule I have only the one test item to work with. The price of this item is 200. I'm not able to have more than one of this item in the cart as the quantity is not available. In order to test out the shipping any further I would need to be able to add 500 or more worth of goods to the cart.
It looks like the currency is correctly set to GBP on your confirm page, how about the pp_confirm page? On the checkout page you will need to adjust line 56 to use GBP instead of USD like this:
$WA_PP_ECO_Set_params[$nextIndex] = "GBP";
You will also need to check on the pp_confirm page to ensure that the currency matches there, if you have any trouble with it post back with a copy of your pp_confirm page.