on the checkout page, look for the following code:
$WA_PP_ECO_Set_params[0][$nextIndex] = "Currency";
$WA_PP_ECO_Set_params[1][$nextIndex] = "USD";
and change it to:
$WA_PP_ECO_Set_params[0][$nextIndex] = "Currency";
$WA_PP_ECO_Set_params[1][$nextIndex] = "GBP";
on the confirm page, change:
$PP_DirectPayment_required[0][$nextIndex] = "Currency";
$PP_DirectPayment_required[1][$nextIndex] = "USD";
to:
$PP_DirectPayment_required[0][$nextIndex] = "Currency";
$PP_DirectPayment_required[1][$nextIndex] = "GBP";
on the pp_confirm page, change:
$WA_PP_ECO_Do_optional[0][$nextIndex] = "Currency";
$WA_PP_ECO_Do_optional[1][$nextIndex] = "USD";
to:
$WA_PP_ECO_Do_optional[0][$nextIndex] = "Currency";
$WA_PP_ECO_Do_optional[1][$nextIndex] = "GBP";