The England Vs. Italy Euro 2012 match hit a peak of 23m viewers in the UK

"England's Euro 2012 exit brought a live match average audience of just over 20 million to BBC1 on Sunday night – the biggest football ratings on any channel since the national team departed from Euro 2004, also after penalties.
BBC1's live match coverage of England's defeat by Italy on penalties, after the match finished 0-0 following extra time, averaged 20.2 million viewers and a 67.8% audience share over two and three quarter hours from 7.45pm.
Viewing hit a five-minute peak of 23.2 million viewers, a 77.4% audience share, towards the end of the match. This was the highest five-minute peak figure for any programme on any channel since England v Portual in Euro 2004.
Full coverage, including pre and post match comment and analysis, averaged 17.4 million and a 62.3% share between 7pm and 10.45pm.
This was the biggest football audience since England went out of Euro 2004 on penalties to Portugal in Euro 2004.
That Thursday night match in June 2004 on BBC1 had a live match average of 20.7 million and a five-minute peak of 24.7 million.
The biggest live match average for a football game in recent times is 23.8 million, for ITV's coverage of England's departure from the 1998 World Cup – once again on penalties, this time to Argentina."
Source:  The Guardian, 25th June 2012
Note - Barb only measures viewing at home - people watching in pubs etc were not counted