Political parties have a bad reputation in the UK. This is not surprising when you consider how they treat their members.
One of the criticisms used against PR is that party leaders will have even more power - to choose candidates and to do 'backroom deals' to fix policies.
In fact it is our present system, first-past-the-post that has led to both more power for party leaders and to more 'backroom deals'.
When choice is limited, competition is restricted. MPs residing in safe seats for life are not accountable to anyone but the party hierarchy.
The Labour party barred people from remaining in the party because they were too 'left wing' and making the party unelectable under fptp. Nobody should be barred from a political party due to their political views.
When Labour threw out left-wing 'entryists' they allowed the party to be taken over by right-wingers and Tory 'entryists'. The democratic way to beat 'entryists' is with debate and to outnumber them in the party.
To throw out people you don't like is the end of a democratic party. Our present electoral system encouraged Labour to become undemocratic, PR would encourage the opposite because no longer would people be restricted to just Tory or Labour, other parties on both the left and right would become electable options that voters could join.
To make parties more democratic we need to pass laws that:-
(i) Only local party members can decide (by ballot) who goes on the candidate list and who becomes their party candidate.
(ii) Anybody on the electoral roll should be allowed to join a UK political party and their voting rights cannot be taken away whatever their political views.
Further it would be great if we introduced 'open-list' PR so that voters can reject parties and candidates that are not democratic. The voters would order the list and decide which candidates are elected.