They are, in fact, on the building. There are numerous friezes representing the law givers. Moses and the 10 Commandments are there. But so are Muhammad, Hammarabi, Confucious ect ect.
The problem is not a religious statue on public property, per se. It is that there is no other statue allowed. THAT is identifying one religion as that of the gov't.
The historical background of what an 'establishment of religion' actually is, has not one thing to do with statues on government property.
It has everything to do with the history of the Anglican Church, whose head is the monarch of England and Scotland and whose clergy are paid by UK citizen taxes.
This whole bruhaha about statues and crosses is just bullshit.