The majority of voters will prefer Tory or Labour or more accurately right wing vs left wing (neither of those are dirty words by the way, extremism is a problem but being left or right is perfectly legitimate). The number of people who are bang in the middle and would flip-flop between the 2 is relatively low. Parties do well at elections when they get all of their side to vote for them, not necessarily when they win votes from the opposition. When Tory voters aren't happy with the Tories they don't vote Labour, they just don't vote or they might vote Lib Dem or someone else "harmless" in their constituency like an independent. When Labour voters aren't happy they don't vote Tory, they vote Green, some local socialist party, Lib Dem or Independent or they don't vote. A huge % of voters will vote for the same party whatever. I think leaders think that % is bigger than it is so think they can get away with more than they can.
Surely for Labour it makes most sense to try to win all of the left by being somewhere between Extreme Left and Centre Left. Move too far left and the centre left stop voting Labour, too centrist and the left stop. You can't ignore one side and just expect them to vote for you. Centre-right policies is suicide for the centre-left just like communist policies would be for the left.
The Tories have less of a problem with losing votes because there aren't as many other options on the right. It's either Tory or don't vote. The key to winning an election for Labour though is getting turnout up for left-wing voters and convincing them to vote Labour, not one of the other left-wing options. My guess is turnout will be lower than recent elections at the next election because a lot of Tories don't vote.
Politics doesn't work if people vote against the party they want to lose. You might get a change of leader but nobody gets what they want. It legitimises policies people don't agree with. Look at how many people trot out nonsense like "left-wing manifestos have been rejected". Labour didn't win under Corbyn but his policies were extremely popular, and would be beneficial for far more people in the country. Corbyn's manifest wasn't rejected, Corbyn was rejected and Labour's Brexit position was rejected.
You should be voting for the party that best represents your views. We only get one chance every few years to indicate what we all want. If we are all playing around and voting tactically then that indication is wrong. If Starmer wants everyone on the left to vote for him then he needs to include them, not exclude them. He needs to include policies that they want other wise he'll find himself winning some swing voters but losing his base and ending up no better off.
The right clearly has almost full control but wants the few remaining lefties got rid of 
