I started out doing the degree (engineering) -> blue chip, management potential, route, but after a couple of years got fed up and left to go windsurfing. Then I tried starting a business and did a few years of low paid but fun/lifestyle work.
At that point I was in my late 20s and in a similar situation to you - i.e. unemployable by anyone I'd consider working for.
So I took out a career development loan and went back to uni to do an MA in public relations in order to make myself more employable in a field I was more interested in.
I then started at the bottom in a PR agency, became a manager and then went to work for a client managing their agencies.
Probably cost me less than your £15K in order to get the qualification and live for a year whilst I did it.
I'm sure I could have done "very well" for myself. HOWEVER...
... after a few years of that I got fed up again, went freelance, started a photo business and spent a lot of time looking after my kids and doing up houses.
Of course it helps that I married a *teacher* who has a steady job that smoothes out the peaks an troughs of our finances (and who doesn't seem to mind me doing things in an unorthodox fashion).
Anyway, the point of this ramble is that yes, £15K is easily enough to set your life off in a different direction. However, you may find that you're just not cut out to do that and you'll end up reverting to type.
But "rapidly approaching 30" is nothing to worry about anyway.
Really it's up to you. You just have to decide what you want to do with your life and get on with it as best you can, but in my opinion you are best off doing stuff you are interested in rather than chasing money.