I don't see an easy way of making such decisions without referendums though since elections are not single issue decisions and since we have fptp to deal with. I think the problem is much wider than just whether the referendum is valid. We need to have proper PR so that govts are not based on minority voting. For any specific constitutional changes, we should have a definition of what requires referendums, and any change from the status quo needs a minimum turnout and a voting threshold.
There is a role for referendums, but they should only happen when there are explicitly clear alternative options, either of which are deliverable.
So good examples are the FPTP vs AV voting referendum - each was a clear option and the government was able to deliver either one. Likewise the London mayor, the creation of Scottish parliament or Welsh assembly. In every case all the discussion on detail had happened in advance and the final agreed and deliverable options were put to the people in a referendum.
The two unsafe examples are the IndyRef, where there was massive uncertainty over what independence would look like, so no clear option and also the Scottish government had no power to deliver some of its stated aspirations, e.g. using the £, remaining in the EU etc. Likewise the EU referendum where the nature of Breixt was (and remains) completely unclear plus again the stated aspirations (e.g. control of immigration, getting trade deals, being in single market etc) are not under the control of the UK government.
In those circumstances (and I said this on the IndyRef as well as the EU ref) you should have a two stage approach with 2 referendums. First a referendum to give a mandate to negotiate an agreement (e.g. on what independence would be like, or an agreed Brexit settlement between the UK and the EU), with all the i's dotted and t's crossed. Then a second (and it could be binding) referendum where that clear and deliverable deal is put to the electorate in a referendum against the status quo. Only in that way can you be sure that there is a mandate for the actual deal on Independence or Brexit.
However, since we won't get that, and since we have had the first referendum, the idea of a second one on the deal is a non starter in terms of its feasibility.
See above - the idea of a second referendum is not a non starter - and certainly if the economic situation deteriorates and public opinion swings against as people begin to realise that the kind of Brexit on offer isn't the kind they envisaged, or wanted, then I think there may be a serious clamour for a second (possible binding) referendum on the agreed deal in, let's say 2019.
If the public haven't changed their mind then there is nothing lost and the deal is securely mandated. If the public have changed their mind how can it be safe to proceed with the most monumental decision the UK has faced for decades against public opinion.