This is from the Authorize call that occurs on the checkout page. In the server behavior for this part you should be able to find where it references USD and update it to GBP. In the result that you posted you are getting these errors:
You do not have permissions to make this API call
It seems that your API user does not have permission for these api calls. You should first check your profile to see if you have any settings that you can update to allow access for these API calls. If you cannot find the setting in your profile that is causing this you will need to contact PayPal.