I found the method in payment so Payment::set_site_currency("GBP"); works but this doesn't change the $ symbol to £. Prices will show as $15.00 (GBP) I haven't been able to find where to change that.

Well spotted, I guess that was moved to payment. Not a bug really although you'd want currency displayed in the catalogue too so perhaps it makes sense to make it a wider, regional setting.
as for the pound sign (don't have one on my keyboard any more) I recall seeing that asked somewhere before, I'll see if I can find it.
I'm keen to try this with 2.3.2-stable so might have a play this weekend.

Using :

Payment::set_site_currency("GBP");
Currency::setCurrencySymbol('£');

changes USD -> GBP and $ -> £ on most pages, but not all areas on the checkout page (see attached "Amount" at the bottom)

Also..

inside the Checkout page there's a hidden form field "Amount" that is vulnerable to people changing the value of the order total (i.e. in Firebug)

and the fix for the bug I posted previously is....
----
sapphire/core/model/fieldtypes/Currency.php - ADD :
static function getCurrencySymbol() {
   return self::$currencySymbol;
}

code/forms/OrderForm.php:70
change '$' to Currency::getCurrencySymbol()