I want preference voting not so much because of its theoretical virtues so much as a way to avoid the stress of tactical voting. Bad enough trying to decide which candidate you prefer, without having to second-guess how everyone else will vote in order to transfer your single vote to the candidate with the best chance of actually winning.
To ease the transition to prefernce voting in British elections I would propose making it optional. Something like the following:
Mark your preferred candidate with a cross X or the figure 1 in the box next to their name.
You may in addition mark with the figures 2, 3, 4, etc., a candidate or candiates to transfer your vote to if your preferred candidate cannot win
In other words deviating from strict ranking as follows:
The idea beaing to allow someone who has not received the memo to vote as if for first-pasth-the-post and for it to have the same meaning as before.
In the spirit of Postel’s Law we can allow further unadvertised slackness:
This is tricker because it means the ballot now presents a sequence of sets of equivalently-preferred candidates. Here is how I think an instant-runoff count would work with paper ballots.
There is a list, initially empty, of eliminated candidates. This is shared by all counters.