I assume that PR means proportional representation. Instant runoff voting does not necessitate proportional representation. You can have an instant-runoff vote

I assume that PR means proportional representation. Instant runoff voting does not necessitate proportional representation. You can have an instant-runoff vote to elect a single MP for your constituency. All you do is rank the candidates in order of preference rather than picking just one of them. There will be one winner, and that's your MP.