Sky has 3.5m HD TV subscribers in the UK

"Digital TV broadcaster Sky has ten million customers following a "strong" first half of its 2010-11 financial year, with high-definition (HD) TV one of the areas to experience growth.
Some 3.5 million people were signed up to the company's HD TV package at the end of December 2010, with growth over the preceding three months up 68 per cent on the same period in 2009.
In total, Sky attracted a net amount of 140,000 new customers over the quarter, while the soon-to-launch Sky Atlantic HD could provide a further boost to subscriptions."

Source:  Cable, 27th January 2011