I suspect they moved it so visitors from London can get off the train and into the BBC building and not have to mingle with us... In and out.
I thought it was fine in Llandaff
The building in Llandaff was spread over two sites, one each side of the road and was getting on for 60 years old. It was unsuitable for modern broadcasting and it was simpler to build a new building than try to modernise the old one.