The result will be the election of top-up MPs chosen from lists of party favourites who have no constituency link. Has anyone thought of the consequences of having a two-tier House of Commons in which the top-up members will inevitably count for less in the eyes of voters and the media?
It is far better to move initially to alternative vote as a precursor to a later move to true PR in the shape of single transferable vote. AV is easily and cheaply introduced, with the same constituencies, candidates and ballot papers, and it gets the electors into the habit of preference voting.
A later move from AV to STV would simply involve the grouping of seats into multi-member constituencies, with an electorate accustomed by then to multi-choice voting.