The AV campaigns (pro and anti) have surely shown that constitutional reform isn't very effectively managed as a party-political football.
In countries with a written constitution, proposals for constitutional amendment are generally treated very differently from the passing of other laws. There are procedures to take the debate to some extent outside the normal run of party politics.
But in the UK, several constitutional changes were made (e.g. fixed-term parliament) when the coalition first came into office without any procedures that would insulate such changes from gerrymandering by the parties; the decision to hold a referendum for AV rather than making that change alongside the other constitutional changes was an ad hoc political one; and most devastatingly there were no procedures in place that might have encouraged the referendum to be conducted with a wealth of neutral information -- for example there was no state-sponsored provision of neutral public-information broadcasting and so forth. We simply don't have a political culture that empowers us to reform a largely decadent political system from outside, rather than allowing its participants to conduct reform in a self-interested way from within.
I don't know how we might go about establishing that kind of external point of leverage, or what form it might take (we wouldn't for example want to give too much power to judicial constitutional oversight). I'm guessing that countries only manage to develop robust written constitutions during (or, rather, immediately after) times of very great crisis -- Germany after the war, for example. Short of very intensive crisis in the UK I think we are completely screwed. British politics is broken.