I thought that the difference between a gun and a howitzer was more to do with the gun carriage being capable of the elevations necessary for high angle fire, and the provision of indirect fire sights and plotting organization, than the gun tube itself.
Granted a howitzer can get away with a shorter barrel, it doesn't necessarily need that, and all that was necessary to convert the 18- pounder to a gun- howitzer would be some moderate amount of carpentry and watchmaking- but some fairly large and painful changes in doctrine. That may be the sticking point.
You can find any number of popular historians willing to say tremendously rude and condemnatory things about the generals of the British Army, but actually looking closely and trying to reconstruct the doctrine of the time, it looks as if the staff were well aware of the potential of trench warfare, and wanted very badly to avoid it.
All the pre war preaching about war being mobile, structured and decisive is basically about winning the war in the open field before it can degenerate into trench fighting and attrition. Optimists like Haig believed it could be done, and had better be- and that preparing for attrition warfare would be a self fulfilling prophecy.
Pessimists- like Kitchener- believed that there could be no quick victory over something as big and professional as the German Army, and they were right, but the British Army chose to kit itself out in search of a quick decision, at first. That's the head of doctrinal pressure a field gun- howitzer in 1909 or thereabouts would be pushing against.