Magento: Setting the Default Currency for a User in Magento

For this add this to the top of /template/directory/currency.phtml

[php]
$currentCurrency = $this->getCurrentCurrencyCode();

if(!isset($currentCurrency)) $currentCurrency = 'NA';

$currencies = array("GBP","USD","EUR");

if(isset($_GET['cy']))
{
if (in_array(strtoupper($_GET['cy']),$currencies)) {
if(strtoupper($_GET['cy']) != $currentCurrency)
{
header("Location: ".$this->helper('directory/url')->getSwitchCurrencyUrl()."currency/".$_GET['cy']);
exit;
}
}
}

[/php]

Then, you can do something with the domains where a .co.uk site will redirect to the .com with ?cy=GBP at the end.