If they make themselves even more economically liberal whilst adopting less controversial social reforms and policy on Irish devolution, such as a lesser peoples budget and negotiating with the Lords instead of all out war, they may be able to steal the Conservatives clothes (Churchill would be an effective leader for this). If they can force a Conservative knee jerk to the left economically by doing this (or at least make it seem so) then the Conservatives might get gobbled up by the emerging Labour party in the twenties.
A quick victorious first world war, whether that's decisive German failures and Allied success or the war being held off until France and Russia are in a much stronger position would also strengthen the Liberal hand. Another landslide in a khaki election after an incredibly victorious war would solidify the Liberals as a party to stay. Maybe even the natural party of Government, although Attlee might have something to say about that.
Thus PM Churchill in the early 1920's with a massive majority after the victorious War of 1917 whilst the Labour and Conservative parties act as a divided opposition.