One of the great things about the British Isles is people aren't afraid to sing. The Welsh in particular are avid singers. They take pride in singing their national anthem well, particularly at rugby matches against teams from other nations.
Those are indeed the players on the field singing their hearts out, not some choir. Can you imagine Adrian Peterson singing our national anthem with the same fervor? Or Joe Mauer?
And then, after the national anthem, they often sing their favorite Welsh hymn, Bread of Heaven, known to Lutherans as "Guide Me O Thou Great Jehovah." What a version! It takes a while to get going, but then it builds. Poor Tom Jones, doesn't know the words.
This is what you call singing lustily. Bread of Heaven often comes out again spontaneously after a victory or during the game after a goal--the entire crowd of 70,000 starts singing a hymn. What a novel idea!
At about 8 p.m. on weekends in certain British pubs, things start getting out of control and people start...singing. Loudly. Hymns. Stephen Foster songs. Whatever. When the pub closes at 11 p.m., they take it to the streets.
We have no such rowdy singing tradition in this country, and it is a pity.
UPDATE: The tradition of singing the Welsh national anthem before rugby matches started in 1905. The New Zealand All Blacks, then, as now, one of the world's greatest rugby teams, would perform their legendary haka, a Maori war chant. Any New Zealander worth their salt can at least to a passable version of the haka.
Rather than stand and stare back without any response, the Welsh decided to sing their national song afterwards, led by the captain of the team.
According to some, it was the first time in the history of any spectator sport that a national anthem was sung before an event. American baseball didn't play the Star Spangled Banner before a game until the World Series of 1918, which was thirteen years before it was adopted as our national anthem.