@Piethagoram The last referendum that was rejected by a 67% majority was AV.

"The Alternative Vote is not a form of proportional representation.

In certain conditions, such as the 2015 General Election, it would have produced a less proportional result than Westminster’s First Past the Post system.

The British public voted not to replace Westminster’s voting system with the Alternative Vote in 2011".

(From Google) 

The referendum has to be to either accept or reject a form of Proportional Representation, probably on a similar model as the PR used for the European Elections, 

The percentage of MPs must mirror the vote share .

I’m not claiming to be an expert in politics, but the First-Past-The-Post (FPTP) system does have the potential to create a robust and steady government. However, it can be disadvantageous for smaller parties.

The problem with smaller parties is that they might lack the experience or have Members of Parliament (MPs) who are capable enough to lead a country. There’s a reason why Labour and the Conservative Party are the two main parties, with the Liberal Democrats being reasonably close behind.

Smaller parties should concentrate on performing well in the areas they represent, while striving to improve in areas where they can. I don’t see many issues with the UK’s political voting system.