So the UK's missiles are not so hot. They accidently shot a US drone, but both missiles missed. Then their trident nuclear missile test failed.

Manufacturers will cut corners and keep more profit if they think they can get away with it. Maybe they thought these missiles will probably never be used anyways, so they cut corners.

Military assets in frequent use work well. US nuclear missiles might have the same problem. So might Russian nuclear missiles. I could easily believe a manufacturer thinking "If a nuclear war starts we are all dead anyways, so why bother to make a functional missile? Maybe evem with a dud missile at least the enemy will survive, so by making a dud I'm kinda saving the world."  This is even more true of bombs since they don't test the warheads anymore.

We might soon find out.