Original: The terms written constitution and codified constitution are often used interchangeably, as are unwritten constitution and uncodified constitution, although this usage is technically inaccurate.
Simplified: There is a debate because the UK has parts of its Constitution codified (written) and uncodified (not written). If the constitution is uncodified, then sources for the agreement are still accessible, they are just written in different various political books or tradition, instead of written all in one place like a codified Constitution.