Ideally the referendum would be an AV format vote with FPTP, AV and PR being the options allowing all those inc myself that want PR a chance to register their preference before voting AV. No matter what your intentions, a no vote to AV will be seen as a vote to keep FPTP. England is so ingrained in the 2 party system that PR is too alien atm.
Give it a few decades of AV and coalition politics and views will change one way or another. The key though is to get AV, however flawed, now which will then let the electorate vote their true intentions before a compromised tactical one.
It would be interesting to see the regional breakdown of votes cast in a 3 way FPTP/AV/PR referendum within the Union to see how the forms of PR already in use in regional government/assembly are perceived.