Here's one way: Labour is voted in, and while proclaiming itself the progressive alternative, continues to do all of the same things as the Tories. This helps persuade the general public that There Is No Alternative, continuing the (decades-long) process of alienating people from political life and persuading them that fighting back in any form is pointless. Several years on, another election takes place. Labour is the "tired" party, and the remaining voter base (mostly varieties of centrist or Tory) decides "it's time for the other lot to have a turn." The other lot, by now, has tacked further right in order to differentiate itself from Labour. The dial of what centrism is slides as a result.
That's not a prediction, that's historical precedent.