C&G is the more technical of the two paths. NVQ is probably as good on the practical side because in that respect both have similar expectations of competence.
At one time you'd have been advised to get work with and work alongside an experienced tradesman, attending college to get the C&G or NVQ part-time. However, the current situation in the construction industry (for site work) and the effect of the credit crunch on the general market for home improvements, makes that an unlikely prospect. In the last twelve months I've known two good joinery businesses (one of them doing a lot of shopfitting) go belly-up, taking nine men down with them. All experienced men, they're all now competing for what's out there.
Sorry to sound so dire, but the situation is dire unless you're already established in a particular specialty, and even then work is down on what it used to be.
You're also painting yourself in a corner by being so specific about the sector you're aiming to join.
As you're intent on shopfitting, I'd suggest you approach local companies and take whatever you can get in that sector. Certainly go and talk to them to find out what the situation is for them. I'd be surprised if what you hear is heartening to someone headed out on the first steps of a chosen career, because the situation in the retail sector is not one conducive to the opening of new shop outlets.
Right, doom and gloom out of the way, things can only get positive!
From the way you talk would I be right in assuming that you have the necessary hand skills to actually do carpentry? If so, and you have family backing, then you could do worse than apply for a full-time college course to get either C&G or NVQ for no other reason than it will give you a buffer that allows time to review the work situation and decide where the opportunities are, the actual paper qualification is, for all practical purposes, irrelevent. You'll hone your skills and pick up some good tips if you actually listen to the instructors and not take the piss out of them (common now, but if you tried that in my day you'd get a slapping).
I think every other chippy (and doubtless the other trades) on here will agree that the fact that you've taken the time and trouble to ask for advice is working in your favour and demonstrates a good work ethic, so you're more than part way to getting where you want to be.
For all that we moan about aspects of the job, we love what we do and consider ourselves lucky to be able to do it. Our moans are not about what we do, just about who we sometimes have to do it for and who gets in the way of our doing it (councils and government regulations spring instantly to mind).
A lot of us started out small by working for friends and family. My particular specialty came about by accident and just grew. The essential thing is to work on your skills and that knowledge associated with whatever field your interest lies in. If you have the basic skills already, then there is no reason why you can't teach yourself all you need to know. Just start small and keep your ambitions modest. If you get good enough you'll be noticed and demand - and how well you meet the challenges that presents - will then determine where you go and how far.
Whatever happens, keep in touch and let us know.