The U.K. functions perfectly well without a Constitution. Israel in any event has a de facto Constitution, in the form of the Basic Laws. If you want a document that is difficult to change, as in the U.S. Constitution, one can stipulate that the Basic Laws can only be modified with an 80-vote supermajority or something like this (perhaps by a national referendum as well).
Law expanding definition of terrorist activity passes first Knesset reading (Haaretz)
from the article: A danger called constitution