Right... open up the page:
WA_eCart/Checkout/paypal.php
Then you will see on lines 46-47:
$PP_DirectPayment_required[0][$nextIndex] = "Currency";
$PP_DirectPayment_required[1][$nextIndex] = "USD";
update it to GBP.
Same thing on pp_confirm.php... lines 286-287:
$WA_PP_ECO_Do_optional[0][$nextIndex] = "Currency";
$WA_PP_ECO_Do_optional[1][$nextIndex] = "USD";
same thing. Then the last place is on the checkout page... lines 55-56 for me but I might have a different version:
$WA_PP_ECO_Set_params[0][$nextIndex] = "Currency";
$WA_PP_ECO_Set_params[1][$nextIndex] = "USD";
same update. That should get your currency working.