It wouldn't really be fair to let her perform it while she's still a contestant, may be sen as producer favoritism.
She's just released the official audio of her new single today, so Christmas special sounds logical if anything.
Just read about her on wikipedia:
-In terms of success, she's had 8 top ten singles, most recent in 2007.
-Nominated for Best British Female at Brits 3 years in a row.
-Groovejet was number 1 in 2000
-Groovejet was first ever song to be played on an iPod!!
-Groovejet is most played song on UK radio for the last decade
-Murder on the Dancefloor was the most played song in Europe in 2002
-Double platinum debut album
-Oddly enough last year she had number 1 singles in Poland and Russia and has become quite a big name in the club/dance scene.
Quite shocked by some of those facts to be fair.