GNVQ = General National Vocational Qualification
RSJ = Rigid Steel Joist (Iris Jay, the builder's friend)
MFI = presumably refers to a former furniture and decorating chain (Tacky? They were vomit-inducing)
ICC = not sure. Wikipedia suggests lots of organisations with these initials.
GCSE = General Certificate of Secondary Education
OTT = Over The Top, indeed.
OU   = Open University