I edited a couple of your post for security reasons since the details are available to anyone online. When I look through the API interactions I can see that you are using Payments Pro and here are the responses you are getting:
Direct Payment response:
This transaction cannot be processed due to an invalid merchant configuration
Express Checkout Process:
Transaction refused because of an invalid argument. See additional error messages for details.
The transaction currency specified must be the same as previously specified.
The invalid merchant configuration message means that you need to adjust your profile on PayPal to allow for Direct Payment API interaction. There may just be a check box somewhere to enable this for you, or you might have a billing agreement that needs to be confirmed.
For the Express Checkout is seems that the Auhtorize call is returning success, but then in the process call you are getting an error regarding currency. On your pp_confirm page the process Express Checkout server behavior is using currencyID="USD", you need to find this and update it to GBP like the Authorize call.