I did my PhD - in Aeronautical Engineering - part time starting aged ~27 and finishing ~35. It started with a burst of enthusiasm, and I finished it in sheer bloodyminded determination to finish.
It was a massively rewarding and life changing thing to do, and I have absolutely no regrets about doing it. It changed me as a person, and changed my professional outlook as well.
So, I'd certainly not talk you out of that.
However, I doubt that many supervisors will accept you with only an ordinary degree - you'll certainly need an honours degree, and many universities and/or supervisors will want to see an MSc as well.
In terms of your skillset - there are two major areas where there are likely to be significant gaps between where you are from your HND and a BEng(Hons). The first is that your maths will probably be a long way behind the standard of an honours graduate, and realistically for a lot of modern aerodynamics research you'll need to be rather beyond that standard anyway. So, the first thing I'd say is that you want to put some serious effort into raising your standards there. Pretty much everybody in higher education uses Stroud's "Mathematics for Engineering Students", which will cover everything you want in one book.
The second is that you won't have done an individual dissertation - the personal research project which is an essential part of any honours or masters level engineering or science degree in the UK. Unless you have equivalent experience in a professional environment, then any prospective PhD supervisor is going to want to see that.
A couple of bits of advice also if they're any use to you:-
- Clearly you want to know your subject. Really know your subject.
- It doesn't really matter what university you do your PhD with so long as it's fairly reputable and you have access to the facilities you need. But it really really matters that you have supervisor with whom you can work well for the whole of the programme of research.
- Good quality wind tunnels are seriously seriously expensive. To get that sort of access needs a lot of money, so you need to be doing this one of:- (1) part time with backing of your employer, (2) with a huge amount of personal money to fund it, or (3) within a well resourced project. (2) is the only one of those which gives you a lot of personal flexibility.
That said, if you have a really good idea, and can really show your ability to deliver on that, there are scholarships available - but don't underestimate the difficulty of getting that scholarship.
Most opportunities get advertised here:-
PhD Programmes, Research Projects & Studentships in the UK & Europe
or here:-
http://www.jobs.ac.uk/phd
Good luck, but your first stage is definitely to get yourself up to BEng(Hons) standard as a minimum, with top-drawer maths, and ideally you want to be targetting a 1st or 2:1 to have much chance of getting into most schemes.
I do supervise PhDs (currently two students on the go at two different UK universities, both in aviation topics), but have given up doing aerodynamics as, frankly, I find it massively less interesting than flight mechanics or human factors. However, I'd be happy to talk through the issues and realities of it further here or offline.
G