The British had 48 tracked carriers for moving guns as prime movers. They were not self propelled artillery, they were designed to transport guns. .

http://en.wikipedia...._Carrier_Mark_I

During 1916 it became clear that in case of a breakthrough, the very purpose of the first tank, the Mark I, artillery would have great trouble following the advancing troops. Any successful offensive would therefore be in danger of stalling immediately. To solve this problem Major Gregg, an engineer working for the main tank producing company Metropolitan, Carriage, Wagon and Finance, proposed to build special mechanised artillery, using parts of the Mark I. The production of a prototype was approved on 5 June 1916; the actual design began in July. The first prototype was ready to participate in the Tank Trials Day on 3 March 1917. An order of fifty vehicles was given to Kitson & Co. in Leeds. Deliveries to the army started in June and ended in July.

The front was an open area with either a 60-pounder (5-inch) field gun or a 6-inch howitzer.

For transporting the gun only the wheels had to be removed from the gun carriage - these were attached to the side of the carrier until needed again. In theory, the field gun could be fired from the vehicle; in reality only the howitzer could be so used. Alternatively the guns could be unloaded through a pivoting cradle assisted by two winding drums driven by the engine. Above the front of the track frame at each side was an armoured cab for the driver on the left and the brakesman on the right.

Many thanks to crickhollow for some very interesting photos and information:

The machine carries either -

(a) One 60 pounder 5-in gun with carriage, wheels and 54 rounds of ammunition. or

(b) One 6-in howitzer complete with carriage and 64 rounds of ammunition,

1918_8_25 British gun carrier Mk I carrying a 6 inch 26cwt. howitzer Somme. August 1918

A suggested into action drill:

The carriage and ordnance move forward on the slide, then the wheels can be attached.  AComing into action could have been quite quick - Halt, carriage / ordnance start to move forward, one man (possibly) two per wheel brings them to the front , attach, commence laying in centre of arc. Machine reverses, track left. Ammunition now available for firing.

Information on the History of 1st Gun Carrier Company Tank Corps July 1917 to May 1918

Many thanks to @Talesofaseadog for pictures from the National Archives.