To co-operate with coronavirus social distancing guidelines, the finals will now take place in a studio and the judges are all sat 2m apart on separate desks.
Diversity dancer and former Britain's Got Talent star Ashley will replace Simon Cowell, 60, for the pre-recorded shows.
Simon broke his back in three places after an electric bicycle accident and will not be taking his usual place on the judging panel.
However always glamorous Amanda will effortlessly slip into his shoes after being the only other original, as she has been on the show since the first series in 2007.
"It makes total sense for Amanda to be head judge with a deciding vote in a deadlock situation," a source told The Sun.
They continued: "She's the only one, other than Simon, who has been there since Day One and has seen it all.
"She cannot wait for the responsibility boost and is certainly going to take it in her stride."
When the show launched in 2007, Simon and Amanda were joined by Piers Morgan, 55, and opera singer Paul Potts was crowned the winner.
Former Mis-Teeq star Alesha and comedian David both joined in 2012, replacing David Hasselhoff, 68, and Michael McIntyre, 44.
And while Simon is missing the semi-finals, he hopes to make a grand return for the live finale, which is set to air in October.
The music mogul was also forced to miss filming for America's Got Talent's finals, and singer Kelly Clarkson took his place.
There are 40 acts appearing in the semi-finals, including the Golden Buzzer performers chosen by Simon, Amanda, Alesha, David and hosts Ant McPartlin and Declan Donnelly.
Simon's act is singer Fayth Ifil, Amanda's is mouther-daughter singing duo Honey and Sammy, and Ant and Dec chose comedy musician Jon Courtenay.
Meanwhile, Alesha put through comedian Nabil Abdulrashid and David picked Manchester choir Sign Along with Us.
Those five acts will be joined by a whole host of other performers hoping they have what it takes to win the glory.
Britain's Got Talent returns to screens Saturday, September 5 at 8pm on ITV