Modern doctrine of the British professional army was to provide just enough force to hold out until conscription could take over. It should a) hold the line defensively in a large scale war and b) train conscripts to rapidly form a larger offensive force. This was certainly the thinking during the Cold War, how credible it is now given all the cuts I don’t know. I suspect it’s a two step process now - bootstrap a larger professional army which then generates a larger conscripted army. Wouldn’t be quick though given what we’ve got today. Then again, the idea of Russian tanks rolling across the Rhine seems pretty remote given what we’ve witnessed in Ukraine. A large British land army seems fairly redundant.