what killed this K-series engine?

gonna put the expertise to the test of members.something i never did solve.
a few years ago i had a little rover 114(metro)with a K-series engine,same as the MGF.
it started and drove well,but when the engine warmed up the oil warning light would start to flicker.my dear hubby figured it was a dodgy sender switch.so he put it to the test,and promptly destroyed the engine,seizing the crank and bending the conrods.
so my question is....what was wrong that the oil supply seemingly failed at operating temperature and not until then.
i learnt all of my engineering know-how from him,and he doesnt know the answer.his opinion is oil-pump failure,but cant explain why it waited til it had warmed up before this would happen.hes built several race-prep engines,even a few competition winning ones,and i know this is a bug bear to him.any ideas folks?

The K series is a law unto itself. It's frankly a disastrous engine and love head gaskets. By knackering it beyond use, you did it a favour.

If a previous owner did poor oil changes, a skim will form in the bottom of the pan that can pulg off the oil screen, thereby stopping all oil flow. (I nearly killed a Ford 400 that way. Oil pump seized, sheared the dist drive gear and stopped the motor before it destructed)