As far as I can tell The Guardian isn't a member of IMPRESS either - at least, they're not on their list of members, and in fact none of the other major newspap

As far as I can tell The Guardian isn't a member of IMPRESS either - at least, they're not on their list of members, and in fact none of the other major newspapers are either. Their complaints page also doesn't mention membership of any independent regulatory body - it suggests complaints are handled entirely in house with no avenue of external appeal or complaint, which is my understanding as well.