this bit isn't relevant to you, but i'll mention for anyone else wanting to add currency to the header in WHMCS directly...
now as I said, that won't apply to you because you're wanting to do this on an external site.. and have it influence a data feed.
fundamentally, you just need to pass a variable value into the feed for the currency - but as i've got no idea if this external site is html, php, wordpress or whatever, i'll give you a basic way to do it in php...
<html>
<body>
<?php
$currency = $_GET['currency'];
echo "<a href=\"".$_SERVER["PHP_SELF"]."?currency=1\">GBP</a> <a href=\"".$_SERVER["PHP_SELF"]."?currency=2\">USD</a><br><br>";
?>
<script language="javascript" src="feeds/productsinfo.php?pid=1&get=price&billingcycle=monthly¤cy=<?php echo $currency ?>"></script>
</body>
</html>
obviously, those links can be replaced with flags, images etc and there are other ways to do this - the point being that you are passing a currency value to the data feed and it is then using that value to pull the correct detail for that currency from the database.