The song contest is being held in the UK for the first time since 1998.
Tonioli has joined the ITV show this year.
The BGT judge and the Ted Lasso star will join Ukrainian singer Julia Sanina to host the semi-finals.
Dixon said she loves Walliams 'very much'.
The judging line-up has had a rare change after David Walliams' exit.
The TV star has been a judge on the show for 10 years.
Morgan weighed in after it was reported that Walliams made unpleasant comments about 'BGT' hopefuls.
The one-off special showcases TV stars' singing skills as they talk about the importance of spending time with others.
Which celebrities have lifted the glitterball - and where are they now?
The cheating scandal came out in 2006.
The TV star's skimpy outfits have been criticised in the past.
Cowell, David Walliams, Alesha Dixon and Amanda Holden were joined by hosts Ant McPartlin and Dec Donnelly at The London Palladium.
The new singing contest is set to launch on ITV.
A big-name line-up features in the new Simon Cowell show.
Alesha Dixon was eight months pregnant when she was fronting 'The Greatest Dancer' with Jordan Banjo.
The Britain's Got Talent judge says she wants her daughters to be able to see her perform on stage.
The mother-of-two said she did not have a bad childhood despite witnessing violence.
Phillips was axed from 'Strictly Come Dancing' in 2009, sparking a huge debate about ageism in TV.
Show bosses originally postponed production on the ITV show.
The 'Britain's Got Talent' star is struggling with the juggle like the rest of us.
The audition stages of the show were set to be filmed at the end of January.
It comes after Diversity's dance routine prompted more than 24,500 complaints.
The routine triggered thousands of complaints to broadcasting watchdog Ofcom.
Ashley Banjo channels Elton John by walking out to I'm Still Standing after Diversity's Britain's Got Talent performance sparks more than 15,000 complaints
There was a mixed response to the troupe's Black Lives Matter-themed routine.