Kurt played eight songs on the stage of the Villiers Theater in London, two cover versions that brought him victory in the Norwegian and World Idol competitions and six of his own compositions.
Nilsen admitted to being nervous and sometimes stumbled over his English but won over the crowd of 200 with his voice and between-song confessions.
"Kurt has written six of the eight songs himself and that shows that even if the Idol format has gotten a lot of criticism, it doesn't matter when we find people like Kurt. This was exceptional," international promotions chief at BMG, Dave Shaz, told newspaper Dagbladet.
"He has a fantastic voice, a huge talent. We are really impressed. Of course the Idol label could count a bit against him but he has grown out the concept. And Kurt is so cute! Not sexy, but a guy you really want to hug," said Giulia Bould and Vicki Robinson from radio station Tower FM to Aftenposten.
Paul Sexton, a music journalist at The Times told newspaper VG that Kurt should work hard on getting exposure in the UK.
"He has an incredible voice and I absolutely believe he has chances with those who are interested in music and not all the hype," Sexton said.
Nilsen told the crowd that he used to play for pizza and beer and never believed that he would make it to London or rise above anything more than playing in a local band.
The day before the concert Nilsen gave 17 radio interviews and hopes to make an impression on the British charts.