Unfortunately the POD is 1914. Having said that the Edwardian UK didn't need a bigger army in peacetime, it needed a better one.Yes, the UK building a much larger army helps them greatly in WW1, and by winning faster, it may, again may, prevent financial ruins. The butterflies are profound on a major UK army expansion say starting in 1905 or 1911.
A good place to start would be the Territorial Force. IOTL it was still equipped with Boer War vintage weapons and it's formations weren't organised on the same pattern as the Regular Army. For example its field artillery batteries had 4 guns instead of 6.
If the 14 infantry divisions in the TF were organised on the same pattern and had the same equipment as the 6 Regular Army infantry divisions that would at least have given the British Army a bigger gun and ammunition making industry plus larger stocks of modern artillery shells at the start of World War II. That would have relieved the shell shortage somewhat and allowed the munitions industry expand at a faster rate after war was declared.
I don't know if its true, but I did read in one book that Lord Haldane wanted a TF of 28 divisions but was only able to get the Cabinet to approve 14. I'm not sure that enough men could have been recruited in peacetime if the money had been made available, but what might be possible is that the 20 home based infantry divisions (6 regular and 14 TF) could be given a 100% reserve of modern equipment to replace losses while industry was mobilised and to facilitate rapid expansion in an emergency.
Whether or not that would shorten the war is another matter.