The reason they are selling it is that it is in a prime location in the biggest Capital city in Europe and will get them a huge amount of sovs.
The place they are building in its stead is:-
Much cheaper and will have far fewer people and no printing presses.
So lots of lolly coming in from the sale; much less going out and they still have the cheek to massively hint for donations/free labour to build the new place.
Just like its big brother in Brooklyn/Warwick, London/Chelmsford is a money realisation scheme.
At least Dick Turpin wore a mask!