As well as the 18 pounders at the Royal Artillery Museum, they also have several other WW1 era exhibits, which i've covered in another thread but added a couple of items here which I didn't get photos of last time. Hope they're of interest
Barrel from a 4.5 inch howitzer - by the time of the Armistice there were almost a thousand in action on the Western Front, and a few were still in use by WW2.
The complete weapon
1917 Ford model T converted into a railway locomotive and used for hauling shells
Model of one of the figures that flank the Royal Artillery memorial at Hyde Park Corner
French 75mm gun - although obsolete by the start of the First World War, the French had so many in service they were successful through sheer numbers. They were also used by the British in small numbers