POD no later than 1920...
A POD before 1920 is not just a tough one to split and destroy the Tory party, but also a very hard one to achieve 25 years of Labour.
If you are looking for an issue to split the Tories over then perhaps the best is Tariff Reform. Joseph Chamberlain split the Conservative-Liberal Unionist coalition over it and the echoes of the tariff question went on into the interwar period. If this doesn't fade away as IOTL but continues to hamper the Conservatives (pushing out moderates like Stanley Baldwin and Bonar Law who had wide electoral appeal in the 1920s and 1930s) then you could see the Tories so divided they split into factions like the Liberals do after their failure at the polls in 1923, leaving Labour the only cohesive national party.
With a Labour Party coming to power as IOTL in 1923 as a minority, facing a weak opposition, the chances are that if they called a snap election in 1924 they might secure a majority. Labour were an untested force in the 1920s and made a lot of people uneasy but were also the embodiement of a lot of hope for a new Britain. If the Tories remain divided and Labour are able to effectively deal with the General Strike in 1926 (which they could probably do well given their links to the TUC) they could well ride a wave of popular support into the 1930s.
The Great Depression would be the hardest thing to maintain a majority for, but there were some radical and impressive ideas on how to affect change in society then (such as Moseley's plan for the economy) that might have seen Labour cling on to public support in an almost New Deal kind of way. If they can cling to power to 1939 they would probably form the same coalition type agreement for national security the tories did OTL. This would take them to 1945 (providing none of this has butterflied WWII) and given the public support OTL for their effective promise to build a better Britain post-war, you could easily see them win a post-war majority at least until 1948.
1923-1948 is twenty five and, by extension, such a long period out of power that the Tories would be reshaped beyond recognition.
For an alternative past your 1920 POD have New Labour stay clear of Iraq, continuing their liberal progressive policies in the 2000s. Have Brown, upon take over, call an election (which he would most likely win), and it wouldn't be too much of a stretch to see 1997-2022 as a Labour quarter century. Remember a lot of political commentators in the early 2000s were talking about the Conservative Party as a place politicians "went to die" and its likely that, with no Cameron-renaissance more would bolt to UKIP as "young and vibrant" alternative. By the late 2010s/early 2020s Labour would probably rely on a coalition with the Greens/LDs but its not inconceivable people would still choose them as the Government come election time. Because Iraq dominates British political memory these days, people forget just how much broad support New Labour had.