It would be fairly safe to say that the vast majority of the audience singing at the top of their lungs in the dark and the rain at Glastonbury do not sing hymns in church. Rock concerts and football/rugby matches probably serve as the last remaining place British 20-somethings experience the power of congregational singing.
I think there are more similarities between this singing and congregational singing than simply a bunch of people singing at the same time.
This struck me a couple of years ago listening to Coldplay's free live album. U2's music is famous for its spiritual leanings and anthemic sing-a-long songs (Pride, Sunday Bloody Sunday, Where the Streets Have No Name, One etc.). I think of Coldplay as a kind of U2-light. They are clearly influenced by the seminal Irish group both in sound -- anthemic songs replacing Edge's sonic pallet with Chris Martin's piano-- and in lyrical content (a little more deeply meaningless perhaps).
Now at Glastonbury 2011, Bono leads his congregation in singing a song from this new hymnody they helped create. Interestingly enough the crowd is able to join in because they are all familiar with the song. To enable congregational singing you need a common musical vocabulary. Also interesting is that as U2 and countless other rock bands know, if you want to move people from being a passive audience to one that sings loudly together with you then in addition to singable choruses, the band has to drop its volume so that people can hear themselves. It is this group/congregational singing that creates the community whether at a U2 concert or in church.