$code=isset($_REQUEST['code'])?$_REQUEST['code']:'GBP';

   $write_connection = Mage::getSingleton('core/resource')->getConnection('core_write');

   $fields_arr = array();

   $fields_arr['quote_currency_code'] = $code;

   $where = $write_connection ->quoteInto('entity_id =?', $shoppingcartid);
  $write_connection->update('sales_flat_quote', $fields_arr, $where);

$quote = Mage::getModel('sales/quote')->load($shoppingcartid);

$quote->setCurrency('GBP');