Well it is unlikely you would see Alan Titchmarsh on it.
Alan left the show as lead presenter a couple of years ago and was replaced by Monty Don as lead presenter.
And if you look at Chris Beardshaw's web site you will see most of his shows are now on ITV so my guess is he was poached by ITV which stops him appearing GW.
http://www.chrisbeardshaw.com/
And RACHEL de Thame (not Rebecca) has FOUR children so probably found it hard to fit in filming GW.
In fact there has been a continous rotation of presenters over the years, as you will see if you watch the old shows on UK TV Gardens channel on Sky.
Plus the show is now an hour so perhaps they decided to broaden it out, rather than the whole hour coming from Barnsdale every week.