HMV had 22% of the UK physical music & DVD market in 2012

"According to Verdict, HMV’s share of the combined music and DVD market, defined as physical and downloaded products bought on and offline, was 22.3 per cent last year. Amazon came in second position with a share of 17 per cent.
Kantar Worldpanel, a consumer research group, has estimated that if HMV disappeared from the high street entertainment retail sales of £300m would be lost – over 9 per cent of the total entertainment market – as a result of fewer sales from browsing and buying on impulse."

Source:  Financial Times, 23rd March 2013

Note - Kantar reported very different market shares for 2011 - 17/5% - so beware...