An increase in public relations and spin doctors to manipulate the public have increased the dominance of the party leaders especially in the role of Prime minister and this role has almost become oligarchic. Leaders have been seen to be decentralising their powers to ordinary members through "Plebiscitary democracy" where the dominance of the leader is increased because ordinary members are more likely to endorse the policies of leader than the activists are.
In policy making, Conservative leaders have total dominance but do have to listen to other important voices to make sure that they are making the right decisions; theses people includes party elite, parliamentarians and grass roots members. The Labour party policy is made by party members in a vote at the annual national conference and is then followed by the leadership.
"Partnership in Power" was a new document adapted where policy is made after two years discussion and then policies are passed on to the National Executive Committee and then discussed at the party conference. Labour leaders have much less dominance over policy making than their Conservative counterparts. In selecting Parliamentary Candidates, leaders from both parties have little dominance over these decisions.
In the Labour party this is done by a system of One-member-one-vote and then lists are presented to the local parties who then shortlist and then the National Executive Committee approve elected candidates. For the Conservatives, Parliamentary Candidates are elected by formal vetting through interviews and a Selection board followed by application to individual constituency parties and they party elites decide who to put forward to a general meeting where a short listed candidate is chosen.
Due to the increase in internal democracy through allowing ordinary members to have a say in thing, it seems that the leaders have become less dominant of their parties but however; this "Plebiscitary democracy" has actually increased the dominance of the leader as it is likely that the ordinary members are more likely to support the leader than activists due to media focus presenting leaders in an attractive way for example Tony Blair viewed as a Christian family man.
Dominance can be dominated by external factors like publicity and electoral success, as with little public support, leaders are unlikely to maintain full party support, as they seem less valuable. Overall, the Conservative leaders have more dominance over their parties than Labour leaders as they have dominance over important areas like policy making even though they seen to have less mandate as they can emerge without proper support from party members and can be challenge more easily.
There is an existence of Oligarchy in the current system but the leaders still have to ensure that they please a certain number of people in order to retain their dominant role e. g. Thatcher who had a large amount of member support but was challenged by other MPs and was successfully removed from power; losing her role altogether.