If we do compare AT&T as a US carrier and Orange UK we will see that their main similarity is the partnership with Microsoft. Those two were the first ones to offer the novelties of WP7 smartphones in their countries.
And while ending up the year Orange UK as a tradition introduces its main best sellers for the year. As it turns out HTC 7Mozart alongside with Nokia N8 is leading the list. The fact of the slow sales of the smartphone among the rest of the carriers in the UK is reasoned that the customers do have a strong association of the Windows Phone 7 with the carrier itself.