Field regiment organisation in 1939 was 2 gun batteries each of 12 guns. RHA regiments were organised in two 8 gun batteries. Guns could be a mixture of 18 pdrs, 18/25 pdrs and 4.5 inch Howitzers although some units in the UK had what ever they could find. After the fall of France, regiments started to receive 25 pdrs and this became the standard field piece.
A new organisation based on 3 gun batteries per regiment was in the pipe line and some newly raised units, unofficially, adopted this ahead of the official change.
The field regiment organisation changed again in 1941 to a new 3 battery organisation. This fitted better with the 3 Brigades of 3 Battalions system in an Infantry Division, allowing each divisional artillery regiment to allot a battery to each battalion. The change over was gradual, some units not raising a third battery until well into 1942. The organisation in an Armoured Division was different.