While I know this thread has just about run its course and Tim's gotten the info he needs, I can't help but add my two bits in about qualifications vs. learning-on-the-job.
I am a product of a course at university and feel I am much better for it so I am a bit biased towards formal qualifications.
However, I have found in my extensive job search for design jobs in the UK, that it really depends on where you want to work. For example, if you want to work for a huge corporate in their in-house design studio, then you have to get through an HR department. Now, HR people often just look at the CVs and bin anyone who doesnt match up to the description advertised for the site. Friends of mine have found that a similiar thing happens in publishing (print design is where my focus lies) as most publishing houses have heavy internal beauracracy.
In terms of getting your foot in the door regarding large coprporates (and perhaps even the larger ad agencies), a qulification is essential if only to place it onto your CV for the HR geeks.
BUT
How you deploy your skills and how that is reflected in your portfolio is just as important. Your portfolio is the thing that will seal the deal for you.
Designing well is a mixture of talent and technical knowlege. If the course is planned and executed correctly, you can develop both at university in an environment that is specifically designed for you to do so. ie: not a working studio where everyone may be too busy to help you.
Please don't get me wrong here, I am not poo pooing "learning by doing", this approach has been taken by thousands and proven to be the correct path for them, I just think that there is a lot to be said about getting a qualification too and this shouldnt be written off (as some people in this thread have).
What I can suggest to you, Tim, is that you stick to the path of wanting to do an HND eventually but you can foreground this with experience gained from working for studios now. I say this because the experience you are getting now is going to be a valuable point of reference for you once you start doing a focussed course and, at the end of it, you would have been able to get more out of it having had the 2 years of practical experience beforehand.
Ok this has been long, sorry for that everyone but I just wanted to weigh-in on this issue.
*waits for the learning-by-doing people to come and tear him apart* ;-)