Too many classifications for different things (salvage, type approval, security, and insurance), leading to confusion all round. And even then they are not being used properly....
The motorhog.co.uk site (the OP) has several cars marked N, but they are using it to mean non-categorised (i.e. none of B,C,D or X, the ABI salvage categories) , not the EC category wording they quoted to Exe (which is for type-approval, not salvage: a QP is M1 in that system not N which is a goods vehicle:
http://en.wikipedia.org/wiki/Directive_2001/116/EC .). Which is not the same as the Bluecycle.com use B,C,D,U,N (where they have used the ABI salvage categories and added N like motorhog but also added a U, which may or may not be an official ABI policy, I don't know but some other sites use U as well). ( Are we all confused yet.....?) And all of those are independent of the Thatcham Security rating (which is N1, N2, T1 etc:
http://www.thatcham.org/abigrouprating/index.jsp?page=343 ). And all nothing to do with motor insurance ratings which are 1-50.
All of which is irrelevant I guess if someone wants to buy it, other than it shows it's not un-insurably damaged. I'll stop now.