The author explains that although the U.S. has a formal single constitutional document and the U.K. does not, both countries have written and unwritten elements in their constitutional systems.
Therefore, the difference is not absolute but rather one of degree.
Option (a) is incorrect — the author acknowledges the difference, but qualifies it as a matter of degree, not insignificance.
Option (c) is incorrect — there is no mention of politicians exaggerating the difference.
Option (d) is incorrect — the difference is definable, and the author defines it in the text.
Thus, (b) is correct as it directly reflects the author’s statement.