Your definition of what constitutes an officer is confused. An officer in the British Army holds a commission, a non-commissioned officer does not and a warrant officer holds a warrant and is drawn from the ranks of the NCO's. Only commissioned officers are considered to be "officers" and everyone else constitutes the "other ranks". As such everyone from a Second Lieutenant through to a Field Marshal is an officer and everyone from private soldier through to Regimental Sergeant Major is an other rank, NCOs are anyone senior than a private soldier.

In terms of the percentage of officers to other ranks less than 10% of the British Army were officers in the First World War (six weeks provides concrete figures which I can look up at a later date) and they had an exceptional high mortality rate particularly in the infantry. The low percentage of officers is not however due to this mortality rate. In an infantry platoon you would have had 1 junior officer to a platoon of 40-50 other ranks. In addition to the platoon commanders you would have had the company commanders, battalion commanders, the regimental commander and a variety of administrative staff posts. Beyond this you have the general staff (everyone from full colonel above) and the numerous staff jobs associated with this. As such there are less officer roles to fill and these were in huge demand throughout the war (although the high mortality rate did lead to a far higher rate of commissions from the other ranks particularly post 1916 who were termed "temporary gentlemen"). It is therefore due to the structure of the British Army which is why you have a small number of officers.

Finally your understanding of CO is again slightly confused. A CO is the commanding officer of formation of battalion level or above, for example you would have CO 1st Battalion Welsh Guards or CO Guards Division. An OC is an officer commanding and would be used at platoon or company level. If you want me to elaborate on anything please ask and I would recommend "six weeks, the short and gallant life of a British officer in the First World War" by John Lewis Stempel.