If you hit a car from behind you are usually legally at fault?

In the UK - the answer generally speaking is "yes". The Highway Code requires you to keep a safe distance behind a vehicle in front such that if that vehicle has to stop - even in an emergency - you have sufficient time to react and stop in time to avoid a collision occurring.

Notably there are a couple of exceptions:

1. In the UK there is an insurance scam starting to develop which involves a vehicle packed with individuals pulling in front of a car and slamming on the brakes to actually cause a collision. in such circumstances and if it can be proved the driver that collides with the one in the front should be found to blame.

2. The motorway scenario - again a vehicle can legitimately pull in front of vehicle in the near lane, but if that vehicle manoeuvres too quickly leaving too small a gap and brakes suddenly it is likely the driver behind will not be found to be At Fault.