Your mortgage protection sum assured is the amount that would be paid out to clear your mortgage if you died, so it must be at least as much as your mortgage. It generally reduces from year to year as the amount you owe on your mortgage goes down. This is called reducing term assurance.
Do I need mortgage protection if I already have life insurance?
Mortgage protection is designed to pay off your mortgage if you die, not to provide a cash sum to your dependants. So, you will usually need separate life insurance to provide for a cash lump sum if you have a dependant