I know that 1st Class degree is 70% +

But how much in percentage would you need to average for a 1st Class degree with distinction?

This is for the UK.

To get what is called a starred first in Cambridge you need to get more than 70% in all of your papers.

Not all Universities offer this grade. The above poster is correct in relation to the History Tripos at Cambridge, but not the Law Tripos. The Law Tripos requires this, and additionally an average of 150 (ie 75%) and above.