Has to be distinctive I suppose - also the desk is for just one presenter and that style wouldn't work elsewhere as regional news and news24 all usually have two presenters.
The main BBC1 newsdesk is more like two small round ones - so that they can still have guests and sports presenter when needed. The ones for News 24 and World (and regions) are more larger and more oval. So for News 24 you can get 4 people on it at once. AFAIK Alastair is filmed as Asa said at Elstree - hence only one window. The newsroom backdrop is available on tape - it was used for a while after the BBC bomb.
As to what Huw does while the regions are on - he goes to the 6 o'clock de-brief in the newsroom (the little summary at the end is produced by a different team - the ones who put together the daytime summaries). Then he waits!