Modern conscription was an innovation of the First World War, though the militia (restricted to domestic employment, AFAIK) had conscription to fill units in the 18th to early 19th Century.

And in any case, I misread parent comment.

So I don't really know the history but I thought British Army planned on mass call-ups either voluntary and then if needed by conscription long before WWI, as mass mobilization isn't something you can do overnight without prep.

I believe Lord Kitchener campaigned for this in WWI until it became clear to the government at large that sustaining an army appropriate to the struggle they had entered was going to be an order more difficult than envisaged.