The supporters of retaining First Past The Post all seem to be supporters of majority government - where only one party is in control of the government and makes all the decisions that face the country. One of their main arguments against changing away from FPTP is that it would increase the chance of coalition governments - where several parties work together, compromise and moderate each other in the national interest.
I have a question for the Conservative and Labour politicians, members, activists and supporters who wish to retain majority government. Why do you want to guarantee that your main political rivals - whose values you oppose - are guaranteed to have periods of unfettered power over the country?
Because this is what supporting majority government guarantees. The option of (a small fraction of) the electorate to kick out unpopular majority governments and replace them with another will inevitably mean at times kicking out your party and installing your rival.
The effect of this means that Labour FPTP supporters are saying that it was right that Thatcher got unmoderated control of the UK in 1979, and Tory FPTP supporters are saying it was right that Blair got unrestrained power in 1997.
And if you don't support that, yet still support majority government? That must mean you want your party to have power over the country forever. That's not democracy, that's dictatorship.