UK drivers, is it always legally the fault of the driver who goes into a car in front, even if the car in front breaks suddenly, yes or no?

Yes. You should always allow a safe distance to make an emergency brake application.

Normally it is, although the magistrates might take a different view if the collision was proved the result of wilful action with the intention of collision, by the driver in front.

There was a spate of false "accidents" with spurious "whiplash injury" claims some years ago, caused by deliberately stopping abruptly enough for imact from the following car; but the Police and insurance companies soon realised these were fraudulent.