Unemployment Only Cover
Unemployment only will cover you if you are made redundant (you also need to be registered with the government as unemployed and you have to be actively seeking work).
Unemployment & Sickness Cover
Accident and sickness will protect you against accidental injury that stops you working and long-term illness (this will have to be certified by a doctor to claim) while accident sickness and unemployment will protect you against both.
ASU Cover
Accident, sickness and unemployment cover as you can gather covers your mortgage protection is a combined policy and covers you in the instance of any of the ASU occurances and is in place for the lifetime of the mortgage.