Despite being originally left out of the 2006 Speedway Grand Prix series, he replaced the retired Tony Rickardsson, eventually winning two Grand Prix meetings and finishing sixth in the final standings. He also became only the third rider to win a GP as a wild card.
In 2007, Andersen became Danish National Champion for the first time. Andersen left his British club, Peterborough Panthers, mid-season in July 2008 after it emerged that the club had not paid him wages amounting to several thousand pounds. Shortly afterwards he signed for Coventry Bees and teamed up with Chris Harris to win the Elite League Pairs Championship. Andersen rode for the Poole Pirates in 2009, and moved on to join the Belle Vue Aces for the 2010 season. He returned to the Peterborough Panthers team towards the end of the 2011 season, replacing Nicki Pedersen. Andersen led Swindon Robins to Elite League glory in 2012, and on 28 January he eventually was able to ride for the Wiltshire side again in 2013 following a winter-long battle with parent club Peterborough.