Checking the Wikipedia UK election pages, Labour got 21% of the popular vote in the OTL 1918 general election. The best Labour results in popular vote before 1945 were 38% in 1935, and 37% in 1929. The Labour Party first got a majority of the seats in the House of Commons in 1945.
IOTL, the Conservatives and the coalition Liberals polled 53% of the vote in 1918, which is 21% greater than Labour, with three quarters of that going to the Conservatives.
By 1918, the best the Central Powers can likely do is the status quo ante bellum in the west, and confirmation of their gains against Russia in the east. The UK loses nothing other than the humiliation of fighting a massive and expensive war over nothing. If do to butterflies, they avoid trying to introduce conscription to Ireland, they wind in a better position in Ireland.
Also, if Lloyd George can't secure victory in World War I, there is a good chance that the Conservatives stop backing him and fight the election on their own. Most of the coalition Liberal MPs go back to the Asquith led Liberals and the Liberals fight the election as a united party. That will make a difference in the results. The two Liberal factions got a combined 26% of the popular vote IOTL in 1918, but I think taking the UK into a losing war and the lack of prestige by Lloyd George, who likely is on the sidelines in the election, would mean a united Liberal Party gets something more like 21%.
But I don't see a Labour majority in the House of Commons in this situation. This is still the first election they are fighting as an organized party independent of the Liberals. I see something more like the 1929 results, with Labour at 37% to 38% to the Tories, and this would still mean almost doubling their OTL 1918 popular vote percentage. This is probably the best possible situation. I think either the Tories still win, albeit more narrowly, or there is a Labour government, but it is a minority government dependent on Asquith.
Also likely after the compromise peace, Lloyd George resigns before the election, and the incumbent PM is Bonar Law, or possibly Bonar Law declines to form a government in this situation, since it only hurts the Conservatives political standing, and Balfour is interim PM.