Your comment assumes that under PR people would vote the same way as they do under FPTP. There are other important factors that could affect the parliamentary make-up should FPTP be replaced by something else such as PR.
Firstly, some voters vote tactically. They don't particularly like Party A, but under FPTP they vote for them rather than for Party B in order to keep B out. Given the choice under PR, these voters might now vote for C, D or E if they felt that their votes would now result in their preferred party being represented. They would have the opportunity to vote for a party, rather than against a party. (In the last GE, barely 50% of those who voted actually voted for one of the two major English parties).
Secondly, almost one-third of the registered electorate does not actually vote. There may be several reasons for this; for example, in a "safe" seat a voter might not bother to vote for any of the other parties because there is no chance of their vote having any influence on the outcome; in a constituency with a two-party split, the voter who supports neither party might likewise abstain, being unwilling to be seen to endorse either candidate; or simply, even where there are several candidates a voter might not vote because their preferred choice is still "none of the above."
Thirdly, it is estimated that around 5 million people who are eligible to vote are not actually registered to do so. Adding these to the electoral roll would add over 10% to the registered electorate. Similar to the abstainers above, there are various reasons why this might be the case, but PR would perhaps encourage more people to register and vote by dispelling the doubts of those who feel that "my vote doesn't count anyway" or "politicians are all the same"