I don't like the standard bling grille on my 2012 4.4 TDV8. Is it possible to change to the early horizontal bar type ?

Original on release 2003 year horizontal one ?

Not really, the bumper / lights profile changed in 2005/6 time frame I think which meant the lights reduced in height and the bumper then made a step up to make up the gap the smaller lights would have left. V8 or else ...

Do you have the chrome grille on your Westminster?

You can swap it for the more discreet standard grille fitted on 2010-2012 model cars:

Lots of variety available if you shop around.
This is my personal favourite.

Click image to enlarge