Why would left wing socialists in the Labour Party have less leverage than if they were outside the party??

We have seen with the Conservative Party that 100 or so Tory "rebels" are able to influence the direction of the Johnson Govt. Why not a similar situation under a Labour Govt? Any Left in the Labour Party still has some access to some basic democratic mechanisms such as the annual conference and elections to the NEC to exercise some influence over a Labour Party Government.

Yes there is a loyalty and discipline issue but if the Left had a distinct enough political philosophy and programme they could act to influence and shape a centrist Labour Govt.

I am actually in favour of PR rather than First Past the Post because one it is obviously far more democratic and more accurately reflects public opinion and two because, what is the great value of electing a "left" government on a minority vote?

We believe real power outside the state rests in the sheer numbers and organisation of the working class and PR would enable this to be more accurately reflected in any parliament.

The case for the Labour Party doesn't rest on this or that electoral system. It is still the mass parliamentary parrty of the organised labour movement with hundreds of thousands of members, millions of affiliated trade unionists, and millions of working class voters who at the very least identify with the class appeal of the name.