The HNC is a recognised qualification, sitting somewhere below a degree but still highly regarded. The fact that the system is changing won't devalue it.
If you look at the RAeS grading guide, you can see that it's regarded as equivalent to an "ordinary" (as opposed to an Honours" degree.
Associate Member (AMRAeS)
There are four ways to qualify as an Associate Member. Candidates must be either:
a. Age 23 and above: Hold a BTEC Higher National certificate, ordinary degree or international equivalent
Have had two years’ practical training
Have at least three years’ experience at a relevant level of responsibility or…
b. Age 23 and above: Hold an Honours degree or equivalent
Have had two years’ professional development or…
c. Age 30 and above: Have ten years’ appropriate experience or…
d. For Licensed Aircraft Engineers: Hold a UK CAA licence with type rating, or international equivalents, or appropriate company approvals
Have at least two years’ experience involving supervisory and/or technical administration
Of course like any other "academic" engineering qualification it prepares you well to then learn how to do an engineering job and get licences in that area, but it doesn't actually qualify you to DO anything.
I believe that if you talk to them, most university engineering departments will also be happy for a good HNC pass to allow you to skip at-least the first year of a degree if that's what you plan to do.
You won't find, by and large, that anybody differentiates strongly between an HNC in Mechanical Engineering, or an HNC in aeronautical engineering - but if you plan to do the latter at degree level, you'll probably have to self-teach quite a lot of aerodynamics if you're hoping to skip the first year of a degree course.
If you're just looking for a job, I doubt you'll struggle to find one with an HNC plus a few years relevant industry experience. Without experience of-course, you're about as employable as any other total newcomer to the aircraft industry - hardly at-all!
G